Remove usage of register keyword in Extension Modules
- [X] closes #26263
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

ASV results show no difference:

```sh
BENCHMARKS NOT SIGNIFICANTLY CHANGED.
```

As I am fairly certain this keyword was being entirely ignored by the compilers anyway (see issue for reasoning)
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26264?src=pr&el=h1) Report
> Merging [#26264](https://codecov.io/gh/pandas-dev/pandas/pull/26264?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7fafb356549f1c52a9896429ce0241487f42bea9?src=pr&el=desc) will **decrease** coverage by `51.25%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26264/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26264?src=pr&el=tree)

```diff
@@             Coverage Diff             @@
##           master   #26264       +/-   ##
===========================================
- Coverage   91.98%   40.72%   -51.26%     
===========================================
  Files         175      175               
  Lines       52386    52386               
===========================================
- Hits        48185    21334    -26851     
- Misses       4201    31052    +26851
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `?` | |
| #single | `40.72% <ø> (-0.15%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26264?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/latex.py](https://codecov.io/gh/pandas-dev/pandas/pull/26264/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvbGF0ZXgucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/io/sas/sas\_constants.py](https://codecov.io/gh/pandas-dev/pandas/pull/26264/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNfY29uc3RhbnRzLnB5) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/core/groupby/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/26264/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9jYXRlZ29yaWNhbC5weQ==) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/tseries/plotting.py](https://codecov.io/gh/pandas-dev/pandas/pull/26264/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvcGxvdHRpbmcucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/tseries/converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/26264/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvY29udmVydGVyLnB5) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/io/formats/html.py](https://codecov.io/gh/pandas-dev/pandas/pull/26264/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvaHRtbC5weQ==) | `0% <0%> (-99.37%)` | :arrow_down: |
| [pandas/io/sas/sas7bdat.py](https://codecov.io/gh/pandas-dev/pandas/pull/26264/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXM3YmRhdC5weQ==) | `0% <0%> (-91.16%)` | :arrow_down: |
| [pandas/io/sas/sas\_xport.py](https://codecov.io/gh/pandas-dev/pandas/pull/26264/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNfeHBvcnQucHk=) | `0% <0%> (-90.1%)` | :arrow_down: |
| [pandas/core/tools/numeric.py](https://codecov.io/gh/pandas-dev/pandas/pull/26264/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdG9vbHMvbnVtZXJpYy5weQ==) | `10.44% <0%> (-89.56%)` | :arrow_down: |
| [pandas/io/s3.py](https://codecov.io/gh/pandas-dev/pandas/pull/26264/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3MzLnB5) | `0% <0%> (-89.48%)` | :arrow_down: |
| ... and [130 more](https://codecov.io/gh/pandas-dev/pandas/pull/26264/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26264?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26264?src=pr&el=footer). Last update [7fafb35...ca4516a](https://codecov.io/gh/pandas-dev/pandas/pull/26264?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26264?src=pr&el=h1) Report
> Merging [#26264](https://codecov.io/gh/pandas-dev/pandas/pull/26264?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7fafb356549f1c52a9896429ce0241487f42bea9?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26264/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26264?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26264      +/-   ##
==========================================
+ Coverage   91.98%   91.98%   +<.01%     
==========================================
  Files         175      175              
  Lines       52386    52379       -7     
==========================================
- Hits        48185    48179       -6     
+ Misses       4201     4200       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.53% <ø> (ø)` | :arrow_up: |
| #single | `40.73% <ø> (-0.14%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26264?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26264/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26264/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.9% <0%> (-0.12%)` | :arrow_down: |
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/26264/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.86% <0%> (ø)` | :arrow_up: |
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/26264/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `83.91% <0%> (+0.27%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26264?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26264?src=pr&el=footer). Last update [7fafb35...94676f3](https://codecov.io/gh/pandas-dev/pandas/pull/26264?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 these were all just added here: https://github.com/pandas-dev/pandas/pull/25784/files

cc @vnlitvin @anmyachev  @WillAyd try running: `asv continuous -f 1.05 origin/master remove-register -b io.csv -a sample_time=2 -a warmup_time=2`. This will give fairly stable results. Ran that and benchmarks still showed no change:

<details>

```sh
$ asv continuous -f 1.05 7fafb356549f1c52a9896429ce0241487f42bea9 a491c2b217b69208e810c23ce6db7a21d033fd06 -b io.csv -a sample_time=2 -a warmup_time=2
· Creating environments
· Discovering benchmarks.
·· Uninstalling from conda-py3.6-Cython-matplotlib-numexpr-numpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt.
·· Installing a491c2b2 <remove-register~2> into conda-py3.6-Cython-matplotlib-numexpr-numpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt..
· Running 36 total benchmarks (2 commits * 1 environments * 18 benchmarks)
[  0.00%] · For pandas commit 7fafb356 <master~2> (round 1/2):
[  0.00%] ·· Building for conda-py3.6-Cython-matplotlib-numexpr-numpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt...
[  0.00%] ·· Benchmarking conda-py3.6-Cython-matplotlib-numexpr-numpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt
[  1.39%] ··· Running (io.csv.ReadCSVCategorical.time_convert_direct--)...
[  5.56%] ··· Running (io.csv.ReadCSVDInferDatetimeFormat.time_read_csv--).
[  6.94%] ··· Running (io.csv.ReadCSVFloatPrecision.time_read_csv--).
[  8.33%] ··· Running (io.csv.ReadCSVFloatPrecision.time_read_csv_python_engine--).
[ 11.11%] ··· Running (io.csv.ReadCSVParseDates.time_baseline--)...
[ 15.28%] ··· Running (io.csv.ReadCSVSkipRows.time_skipprows--)..
[ 18.06%] ··· Running (io.csv.ReadUint64Integers.time_read_uint64--)...
[ 22.22%] ··· Running (io.csv.ToCSV.time_frame--).
[ 23.61%] ··· Running (io.csv.ToCSVDatetime.time_frame_date_formatting--)...
[ 25.00%] · For pandas commit a491c2b2 <remove-register~2> (round 1/2):
[ 25.00%] ·· Building for conda-py3.6-Cython-matplotlib-numexpr-numpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt...
[ 25.00%] ·· Benchmarking conda-py3.6-Cython-matplotlib-numexpr-numpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt
[ 26.39%] ··· Running (io.csv.ReadCSVCategorical.time_convert_direct--)...
[ 30.56%] ··· Running (io.csv.ReadCSVDInferDatetimeFormat.time_read_csv--).
[ 31.94%] ··· Running (io.csv.ReadCSVFloatPrecision.time_read_csv--).
[ 33.33%] ··· Running (io.csv.ReadCSVFloatPrecision.time_read_csv_python_engine--).
[ 36.11%] ··· Running (io.csv.ReadCSVParseDates.time_baseline--)...
[ 40.28%] ··· Running (io.csv.ReadCSVSkipRows.time_skipprows--)..
[ 43.06%] ··· Running (io.csv.ReadUint64Integers.time_read_uint64--)...
[ 47.22%] ··· Running (io.csv.ToCSV.time_frame--).
[ 48.61%] ··· Running (io.csv.ToCSVDatetime.time_frame_date_formatting--)...
[ 50.00%] · For pandas commit a491c2b2 <remove-register~2> (round 2/2):
[ 50.00%] ·· Benchmarking conda-py3.6-Cython-matplotlib-numexpr-numpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt
[ 51.39%] ··· io.csv.ReadCSVCategorical.time_convert_direct                                                                 42.1±0.5ms
[ 52.78%] ··· io.csv.ReadCSVCategorical.time_convert_post                                                                     59.2±1ms
[ 54.17%] ··· io.csv.ReadCSVComment.time_comment                                                                            28.8±0.2ms
[ 55.56%] ··· io.csv.ReadCSVDInferDatetimeFormat.time_read_csv                                                                      ok
[ 55.56%] ··· ======================= ============ ============= =============
              --                                       format
              ----------------------- ----------------------------------------
               infer_datetime_format     custom       iso8601         ymd
              ======================= ============ ============= =============
                        True           6.38±0.1ms   2.71±0.07ms   2.69±0.06ms
                       False            108±2ms     2.17±0.09ms    2.12±0.1ms
              ======================= ============ ============= =============

[ 56.94%] ··· io.csv.ReadCSVFloatPrecision.time_read_csv                                                                            ok
[ 56.94%] ··· ===== ============= ============= ================ ============= ============= ================
              --                                    decimal / float_precision
              ----- -----------------------------------------------------------------------------------------
               sep     . / None      . / high    . / round_trip     _ / None      _ / high    _ / round_trip
              ===== ============= ============= ================ ============= ============= ================
                ,    2.01±0.07ms   1.89±0.08ms     2.92±0.1ms      2.32±0.1ms   2.35±0.06ms    2.34±0.06ms
                ;    2.22±0.04ms   2.07±0.04ms     2.94±0.1ms     2.29±0.05ms   2.28±0.07ms    2.20±0.03ms
              ===== ============= ============= ================ ============= ============= ================

[ 58.33%] ··· io.csv.ReadCSVFloatPrecision.time_read_csv_python_engine                                                              ok
[ 58.33%] ··· ===== ============= ============= ================ ============= ============= ================
              --                                    decimal / float_precision
              ----- -----------------------------------------------------------------------------------------
               sep     . / None      . / high    . / round_trip     _ / None      _ / high    _ / round_trip
              ===== ============= ============= ================ ============= ============= ================
                ,    4.51±0.07ms   4.61±0.08ms    4.57±0.05ms     3.99±0.03ms   4.08±0.07ms     4.02±0.1ms
                ;     4.56±0.1ms   4.70±0.06ms    4.62±0.04ms     4.12±0.05ms   4.07±0.02ms     4.01±0.1ms
              ===== ============= ============= ================ ============= ============= ================

[ 59.72%] ··· io.csv.ReadCSVMemoryGrowth.mem_parser_chunks                                                                           0
[ 61.11%] ··· io.csv.ReadCSVParseDates.time_baseline                                                                       2.40±0.03ms
[ 62.50%] ··· io.csv.ReadCSVParseDates.time_multiple_date                                                                  2.88±0.03ms
[ 63.89%] ··· io.csv.ReadCSVParseSpecialDate.time_read_special_date                                                                 ok
[ 63.89%] ··· ======== ============
               param1
              -------- ------------
                 mY     52.3±0.6ms
                mdY     21.8±0.4ms
                 hm      431±8ms
              ======== ============

[ 65.28%] ··· io.csv.ReadCSVSkipRows.time_skipprows                                                                                 ok
[ 65.28%] ··· ========== ============
               skiprows
              ---------- ------------
                 None     20.3±0.9ms
                10000     13.6±0.2ms
              ========== ============

[ 66.67%] ··· io.csv.ReadCSVThousands.time_thousands                                                                                ok
[ 66.67%] ··· ===== ============ ============
              --            thousands
              ----- -------------------------
               sep      None          ,
              ===== ============ ============
                ,    16.2±0.1ms   17.2±0.7ms
                |    16.1±0.2ms   17.7±0.5ms
              ===== ============ ============

[ 68.06%] ··· io.csv.ReadUint64Integers.time_read_uint64                                                                    3.83±0.1ms
[ 69.44%] ··· io.csv.ReadUint64Integers.time_read_uint64_na_values                                                          7.24±0.3ms
[ 70.83%] ··· io.csv.ReadUint64Integers.time_read_uint64_neg_values                                                         6.63±0.3ms
[ 72.22%] ··· io.csv.ToCSV.time_frame                                                                                               ok
[ 72.22%] ··· ======= ============
                kind
              ------- ------------
                wide   147±0.9ms
                long   234±0.8ms
               mixed   30.1±0.6ms
              ======= ============

[ 73.61%] ··· io.csv.ToCSVDatetime.time_frame_date_formatting                                                               18.9±0.7ms
[ 75.00%] ··· io.csv.ToCSVDatetimeBig.time_frame                                                                                    ok
[ 75.00%] ··· ======== ============
                obs
              -------- ------------
                1000    6.34±0.1ms
               10000    54.3±0.7ms
               100000    555±7ms
              ======== ============
.
[ 75.00%] · For pandas commit 7fafb356 <master~2> (round 2/2):
[ 75.00%] ·· Building for conda-py3.6-Cython-matplotlib-numexpr-numpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt...
[ 75.00%] ·· Benchmarking conda-py3.6-Cython-matplotlib-numexpr-numpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt
[ 76.39%] ··· io.csv.ReadCSVCategorical.time_convert_direct                                                                 44.2±0.6ms
[ 77.78%] ··· io.csv.ReadCSVCategorical.time_convert_post                                                                     60.7±2ms
[ 79.17%] ··· io.csv.ReadCSVComment.time_comment                                                                            29.0±0.8ms
[ 80.56%] ··· io.csv.ReadCSVDInferDatetimeFormat.time_read_csv                                                                      ok
[ 80.56%] ··· ======================= ============ ============= =============
              --                                       format
              ----------------------- ----------------------------------------
               infer_datetime_format     custom       iso8601         ymd
              ======================= ============ ============= =============
                        True           6.63±0.1ms   2.65±0.01ms   2.74±0.03ms
                       False            109±3ms      2.22±0.2ms    2.01±0.2ms
              ======================= ============ ============= =============

[ 81.94%] ··· io.csv.ReadCSVFloatPrecision.time_read_csv                                                                            ok
[ 81.94%] ··· ===== ============= ============= ================ ============ ============= ================
              --                                   decimal / float_precision
              ----- ----------------------------------------------------------------------------------------
               sep     . / None      . / high    . / round_trip    _ / None      _ / high    _ / round_trip
              ===== ============= ============= ================ ============ ============= ================
                ,    1.95±0.03ms   1.91±0.03ms     2.76±0.1ms     2.21±0.1ms   2.13±0.06ms    2.32±0.07ms
                ;    2.06±0.01ms   2.03±0.04ms    2.82±0.03ms     2.19±0.1ms    2.12±0.3ms    2.16±0.09ms
              ===== ============= ============= ================ ============ ============= ================

[ 83.33%] ··· io.csv.ReadCSVFloatPrecision.time_read_csv_python_engine                                                              ok
[ 83.33%] ··· ===== ============= ============= ================ ============= ============= ================
              --                                    decimal / float_precision
              ----- -----------------------------------------------------------------------------------------
               sep     . / None      . / high    . / round_trip     _ / None      _ / high    _ / round_trip
              ===== ============= ============= ================ ============= ============= ================
                ,     4.34±0.2ms    4.57±0.1ms    4.53±0.08ms     4.03±0.05ms    4.21±0.1ms     4.02±0.1ms
                ;    4.64±0.08ms   4.72±0.05ms    4.60±0.08ms     4.11±0.09ms   4.13±0.05ms    3.94±0.05ms
              ===== ============= ============= ================ ============= ============= ================

[ 84.72%] ··· io.csv.ReadCSVMemoryGrowth.mem_parser_chunks                                                                           0
[ 86.11%] ··· io.csv.ReadCSVParseDates.time_baseline                                                                       2.34±0.02ms
[ 87.50%] ··· io.csv.ReadCSVParseDates.time_multiple_date                                                                  2.92±0.05ms
[ 88.89%] ··· io.csv.ReadCSVParseSpecialDate.time_read_special_date                                                                 ok
[ 88.89%] ··· ======== ============
               param1
              -------- ------------
                 mY     52.7±0.4ms
                mdY     21.7±0.2ms
                 hm      418±8ms
              ======== ============

[ 90.28%] ··· io.csv.ReadCSVSkipRows.time_skipprows                                                                                 ok
[ 90.28%] ··· ========== ============
               skiprows
              ---------- ------------
                 None     19.4±0.4ms
                10000     13.3±0.2ms
              ========== ============

[ 91.67%] ··· io.csv.ReadCSVThousands.time_thousands                                                                                ok
[ 91.67%] ··· ===== ============ ============
              --            thousands
              ----- -------------------------
               sep      None          ,
              ===== ============ ============
                ,    16.6±0.7ms   17.4±0.5ms
                |    16.2±0.1ms   17.6±0.4ms
              ===== ============ ============

[ 93.06%] ··· io.csv.ReadUint64Integers.time_read_uint64                                                                   3.64±0.05ms
[ 94.44%] ··· io.csv.ReadUint64Integers.time_read_uint64_na_values                                                          6.75±0.4ms
[ 95.83%] ··· io.csv.ReadUint64Integers.time_read_uint64_neg_values                                                         6.44±0.1ms
[ 97.22%] ··· io.csv.ToCSV.time_frame                                                                                               ok
[ 97.22%] ··· ======= ============
                kind
              ------- ------------
                wide    145±4ms
                long    225±4ms
               mixed   29.9±0.3ms
              ======= ============

[ 98.61%] ··· io.csv.ToCSVDatetime.time_frame_date_formatting                                                               18.3±0.3ms
[100.00%] ··· io.csv.ToCSVDatetimeBig.time_frame                                                                                    ok
[100.00%] ··· ======== =============
                obs
              -------- -------------
                1000    6.25±0.09ms
               10000     54.1±0.5ms
               100000     548±2ms
              ======== =============

.
BENCHMARKS NOT SIGNIFICANTLY CHANGED.
```

</details>

FWIW gcc at least also discourages the use of the register keyword for local variables:

https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html#Local-Register-Variables
 I ran a test with `-f 1.01` and did not get any changes either. Apparently in the past something misled us, because now it looks completely unnecessary. thanks @WillAyd  > Apparently in the past something misled us, because now it looks completely unnecessary.

My guess is it was relevant for Windows + Python 2 (this combo used VS2008 to build), more modern compilers might be caching stuff to registers more eagerly.

That said, @WillAyd did you run some benchmarks on Windows?