CLN: Remove PY2/3 references io directory
xref #25725
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25886?src=pr&el=h1) Report
> Merging [#25886](https://codecov.io/gh/pandas-dev/pandas/pull/25886?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d40446044fc240f4ec6904d12a7821f0091f3b29?src=pr&el=desc) will **increase** coverage by `0.13%`.
> The diff coverage is `94.87%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25886/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25886?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25886      +/-   ##
==========================================
+ Coverage   91.48%   91.62%   +0.13%     
==========================================
  Files         175      175              
  Lines       52885    52777     -108     
==========================================
- Hits        48380    48355      -25     
+ Misses       4505     4422      -83
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.18% <94.87%> (+0.13%)` | :arrow_up: |
| #single | `41.88% <58.97%> (+0.05%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25886?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/excel/\_base.py](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2V4Y2VsL19iYXNlLnB5) | `92.89% <ø> (+0.4%)` | :arrow_up: |
| [pandas/io/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3B5dGFibGVzLnB5) | `90.23% <100%> (+0.03%)` | :arrow_up: |
| [pandas/io/formats/format.py](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZm9ybWF0LnB5) | `97.99% <100%> (ø)` | :arrow_up: |
| [pandas/io/formats/printing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvcHJpbnRpbmcucHk=) | `85.63% <100%> (+0.29%)` | :arrow_up: |
| [pandas/io/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2NvbW1vbi5weQ==) | `91.91% <100%> (+19.05%)` | :arrow_up: |
| [pandas/io/sas/sas\_xport.py](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNfeHBvcnQucHk=) | `90.09% <100%> (-0.05%)` | :arrow_down: |
| [pandas/io/s3.py](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3MzLnB5) | `89.47% <100%> (+3.11%)` | :arrow_up: |
| [pandas/io/formats/terminal.py](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvdGVybWluYWwucHk=) | `34.78% <100%> (+2.25%)` | :arrow_up: |
| [pandas/io/packers.py](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhY2tlcnMucHk=) | `88.08% <100%> (-0.04%)` | :arrow_down: |
| [pandas/io/clipboards.py](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2NsaXBib2FyZHMucHk=) | `100% <100%> (ø)` | :arrow_up: |
| ... and [5 more](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25886?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/25886?src=pr&el=footer). Last update [d404460...8f7b01e](https://codecov.io/gh/pandas-dev/pandas/pull/25886?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/25886?src=pr&el=h1) Report
> Merging [#25886](https://codecov.io/gh/pandas-dev/pandas/pull/25886?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/31b4019ef290bc55855e079b2b7dddcc39a16fc4?src=pr&el=desc) will **increase** coverage by `0.2%`.
> The diff coverage is `95.45%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25886/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25886?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##           master   #25886     +/-   ##
=========================================
+ Coverage   91.56%   91.77%   +0.2%     
=========================================
  Files         175      175             
  Lines       52775    52607    -168     
=========================================
- Hits        48324    48278     -46     
+ Misses       4451     4329    -122
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.32% <93.18%> (+0.2%)` | :arrow_up: |
| #single | `41.9% <65.9%> (+0.01%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25886?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/excel/\_base.py](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2V4Y2VsL19iYXNlLnB5) | `92.89% <ø> (+0.4%)` | :arrow_up: |
| [pandas/io/formats/console.py](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvY29uc29sZS5weQ==) | `78.12% <100%> (ø)` | :arrow_up: |
| [pandas/io/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2NvbW1vbi5weQ==) | `91.87% <100%> (+19%)` | :arrow_up: |
| [pandas/io/excel/\_xlrd.py](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2V4Y2VsL194bHJkLnB5) | `94.02% <100%> (+0.09%)` | :arrow_up: |
| [pandas/io/formats/terminal.py](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvdGVybWluYWwucHk=) | `66.66% <100%> (+34.95%)` | :arrow_up: |
| [pandas/io/packers.py](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhY2tlcnMucHk=) | `88.08% <100%> (-0.04%)` | :arrow_down: |
| [pandas/io/formats/csvs.py](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvY3N2cy5weQ==) | `98.2% <100%> (-0.02%)` | :arrow_down: |
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | `95.96% <100%> (ø)` | :arrow_up: |
| [pandas/io/excel/\_util.py](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2V4Y2VsL191dGlsLnB5) | `87.67% <100%> (+8.84%)` | :arrow_up: |
| [pandas/io/s3.py](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3MzLnB5) | `89.47% <100%> (+3.11%)` | :arrow_up: |
| ... and [12 more](https://codecov.io/gh/pandas-dev/pandas/pull/25886/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25886?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/25886?src=pr&el=footer). Last update [31b4019...ad97232](https://codecov.io/gh/pandas-dev/pandas/pull/25886?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback : I left the `UnicodeReader` / `UnicodeWriter` classes alone for the reasons I described [above](https://github.com/pandas-dev/pandas/pull/25886#discussion_r269351175).  PTAL. @jreback : Addressed all comments, and everything is green.  PTAL. thanks @gfyoung very nice!