Start porting offsets to cython
This is _mostly_ cut/paste, avoiding moving any of the classes for the time being.  There is a small change in `_to_dt64` that will be described in a comment below.

Defined `dtstrings` for flake8 cleanup in setup.


CircleCI test errors look unrelated.  Are they showing up elsewhere? # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=h1) Report
> Merging [#17830](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5bf7f9a4ff6968fd444fa7098f8dc95586591994?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17830/graphs/tree.svg?height=150&token=eZ4WkYLtcO&width=650&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17830      +/-   ##
==========================================
- Coverage   91.23%   91.21%   -0.03%     
==========================================
  Files         163      163              
  Lines       50105    49909     -196     
==========================================
- Hits        45715    45523     -192     
+ Misses       4390     4386       -4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.01% <100%> (-0.02%)` | :arrow_down: |
| #single | `40.2% <60%> (-0.18%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97.15% <100%> (-0.01%)` | :arrow_down: |
| [pandas/tseries/frequencies.py](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvZnJlcXVlbmNpZXMucHk=) | `96% <100%> (-0.11%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.42% <0%> (-1.39%)` | :arrow_down: |
| [pandas/core/dtypes/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbmNhdC5weQ==) | `98.26% <0%> (-0.87%)` | :arrow_down: |
| [pandas/core/dtypes/missing.py](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL21pc3NpbmcucHk=) | `90.44% <0%> (-0.18%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `92.03% <0%> (-0.17%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.74% <0%> (-0.11%)` | :arrow_down: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.89% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `94.38% <0%> (-0.07%)` | :arrow_down: |
| ... and [26 more](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17830?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/17830?src=pr&el=footer). Last update [5bf7f9a...6118488](https://codecov.io/gh/pandas-dev/pandas/pull/17830?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/17830?src=pr&el=h1) Report
> Merging [#17830](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/44893893db479894a156dccb28c56f7087b32e14?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17830/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17830      +/-   ##
==========================================
- Coverage   91.24%   91.22%   -0.03%     
==========================================
  Files         163      163              
  Lines       50168    50083      -85     
==========================================
- Hits        45777    45686      -91     
- Misses       4391     4397       +6
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.03% <100%> (-0.02%)` | :arrow_down: |
| #single | `40.23% <60%> (-0.1%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/frequencies.py](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvZnJlcXVlbmNpZXMucHk=) | `96% <100%> (-0.11%)` | :arrow_down: |
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97.15% <100%> (-0.01%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.5% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17830?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/17830?src=pr&el=footer). Last update [4489389...ced2c8d](https://codecov.io/gh/pandas-dev/pandas/pull/17830?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 This can be cut down if the diff is too large for easy review.

Looking forward to getting the `dtstrings` alias in setup.py --> fix a bunch of flake8 warnings. also pls run the perf for timeseries & offsets and report any issues. should slightly speed things up. be careful with the cacheable stuff though. Hello @jbrockmendel! Thanks for updating the PR.









Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on October 28, 2017 at 06:04 Hours UTC ```
asv continuous -E virtualenv -f 1.1 master HEAD -b timeseries
[...]
       before           after         ratio
     [3ba2cfff]       [92d7e290]
+        544±30ms         665±50ms     1.22  timeseries.ToDatetime.time_format_no_exact
+     16.6±0.02μs      19.6±0.09μs     1.18  timeseries.AsOf.time_asof_single_early
-     3.75±0.07ms         3.38±0ms     0.90  timeseries.DatetimeIndex.time_normalize
-        449±20ms        400±0.6ms     0.89  timeseries.SemiMonthOffset.time_end_decr_rng
-        20.7±3μs      18.5±0.07μs     0.89  timeseries.Offsets.time_timeseries_day_apply
-        17.9±3μs       15.7±0.1μs     0.88  timeseries.Offsets.time_custom_bday_apply
-      3.70±0.2ms       3.05±0.1ms     0.82  timeseries.AsOf.time_asof_nan
-        29.1±7μs      23.2±0.05μs     0.80  timeseries.Offsets.time_custom_bday_cal_incr
-         141±2μs       112±0.02μs     0.80  timeseries.DatetimeIndex.time_unique
-        604±10μs        473±0.6μs     0.78  timeseries.DatetimeIndex.time_reset_index_tz
-         469±5μs        362±0.2μs     0.77  timeseries.DatetimeIndex.time_reset_index
-       429±0.8μs        331±0.8μs     0.77  timeseries.DatetimeIndex.time_timeseries_is_month_start
-      12.2±0.7ms      9.18±0.02ms     0.75  timeseries.ResampleSeries.time_period_downsample_mean
-       199±0.9μs          149±2μs     0.75  timeseries.Offsets.time_custom_bmonthend_incr
-        224±10ms         164±10ms     0.73  timeseries.ToDatetime.time_iso8601_tz_spaceformat
-      3.12±0.2ms         2.20±0ms     0.71  timeseries.ResampleDataFrame.time_min_string
-        2.86±0ms      2.01±0.05ms     0.70  timeseries.ResampleDataFrame.time_mean_numpy
-      8.48±0.5μs      5.88±0.01μs     0.69  timeseries.DatetimeIndex.time_timestamp_tzinfo_cons
-        44.1±3μs       30.3±0.2μs     0.69  timeseries.Offsets.time_custom_bday_cal_incr_neg_n
-        41.0±1μs       28.2±0.1μs     0.69  timeseries.Offsets.time_custom_bday_cal_decr
-     6.12±0.05ms      4.19±0.05ms     0.68  timeseries.ResampleSeries.time_timestamp_downsample_mean
-           2.37s            1.57s     0.66  timeseries.Iteration.time_iter_periodindex
-       237±0.4μs        154±0.2μs     0.65  timeseries.Offsets.time_custom_bmonthbegin_incr_n
-     8.34±0.01ms      5.40±0.08ms     0.65  timeseries.DatetimeIndex.time_infer_freq_daily
-        608±20ms        388±0.4ms     0.64  timeseries.SeriesArithmetic.time_add_offset_slow
-      43.5±0.8μs       27.6±0.1μs     0.63  timeseries.Offsets.time_custom_bday_decr
-        23.3±3ms      14.6±0.09ms     0.63  timeseries.Iteration.time_iter_periodindex_preexit
-        717±50ms        446±0.7ms     0.62  timeseries.Iteration.time_iter_datetimeindex
-       85.5±10μs       52.5±0.4μs     0.61  timeseries.SemiMonthOffset.time_begin_decr
-        87.3±3μs      53.2±0.05μs     0.61  timeseries.SemiMonthOffset.time_begin_decr_n
-        753±50ms        419±0.6ms     0.56  timeseries.SemiMonthOffset.time_begin_apply_index
-       769±0.6ms        398±0.9ms     0.52  timeseries.SemiMonthOffset.time_begin_incr_rng
-      16.9±0.1ms      8.40±0.01ms     0.50  timeseries.Iteration.time_iter_datetimeindex_preexit
-     5.31±0.02ms         2.51±0ms     0.47  timeseries.AsOf.time_asof_nan_single
-        248±60ms       8.65±0.2ms     0.03  timeseries.SeriesArithmetic.time_add_offset_fast
-        307±40ms      3.81±0.01ms     0.01  timeseries.SeriesArithmetic.time_add_offset_delta
```

About to re-run with CPU affinity to be on the safe side. what were these doing to be so before?

> -        248±60ms       8.65±0.2ms     0.03  timeseries.SeriesArithmetic.time_add_offset_fast
-        307±40ms      3.81±0.01ms     0.01  timeseries.SeriesArithmetic.time_add_offset_delta @jbrockmendel pls always always rebase on master before you run a benchmark. this is against a commit from 10/9. Pretty much anytime you do anything you should rebase. Woops, good catch.  Re-running now. ```
taskset 8 asv continuous -E virtualenv -f 1.1 master HEAD -b timeseries
[...]
       before           after         ratio
     [1977362f]       [92d7e290]
+      27.6±0.1μs       32.0±0.2μs     1.16  timeseries.Offsets.time_custom_bday_cal_decr
-     8.95±0.03μs      8.07±0.05μs     0.90  timeseries.Offsets.time_timeseries_year_apply
-     11.4±0.06μs      10.2±0.06μs     0.90  timeseries.Offsets.time_timeseries_year_incr
-         175±1μs        153±0.7μs     0.87  timeseries.Offsets.time_custom_bmonthbegin_incr_n
-     6.80±0.03μs         5.84±0μs     0.86  timeseries.DatetimeIndex.time_timestamp_tzinfo_cons
-        109±20ms           40.9ms     0.37  timeseries.DatetimeIndex.time_dti_factorize
-      105±0.04ms         36.0±3ms     0.34  timeseries.DatetimeIndex.time_dti_tz_factorize
-           1.31s          299±3ms     0.23  timeseries.AsOfDataFrame.time_asof_nan
-           1.99s          290±3ms     0.15  timeseries.AsOfDataFrame.time_asof
```

(re-running)

The most commonly-called function that got moved to cython is `_is_normalized`, which gets called in `onOffset`, which in turn gets called in `FooOffset.apply` and `offsets.generate_range`.  I'm not sure about that `factorize` and `asof`s, but that would explain (all but one of) the Offsets timings here. Same results again.  I'll dig into the `custom_bday_apply` to see what the deal is. ok this looks fine. ping after perf review (though that looks like a minor issue). Using cProfile I get a tiny speedup with the new version.  With %timeit I get results similar to asv but a narrower difference.  This is a mystery I'm OK with, given that follow-ons are going to optimize the tar out of these. thanks @jbrockmendel 

> Using cProfile I get a tiny speedup with the new version. With %timeit I get results similar to asv but a narrower difference. This is a mystery I'm OK with, given that follow-ons are going to optimize the tar out of these.

I wouldn't have expected much as these are mostly used in scalar type things which are 1-offs; and these are not typed much either (so that *may* help) @jbrockmendel Is there a reason that CacheableOffset and WeekDay are no longer public in `tseries.offsets` ? 
They are not useful? For those using it, what is the alternative? WeekDay is just an enum map and CacheableOffest is not usable on its own
both of these are private classes  OK (note they are *not* private in the sense they are non-underscored classes in a module that is publicly exposed, but indeed seems like deprecation is not really needed in this case)