move and de-privatize _localize_pydatetime, move shift_day to liboffsets
The only non-test place where tslib._localize_pydatetime is used is in tseries.offsets.  This moves that func to liboffsets, de-privatizes it, and moves tseris.offsets.shift_day up to liboffsets.

To avoid import proliferation this keeps localize_pydatetime in the tslib namespace so the tests can get at it.

Adds some typing to localize_pydatetime and fleshes out the docstring.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21691?src=pr&el=h1) Report
> Merging [#21691](https://codecov.io/gh/pandas-dev/pandas/pull/21691?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ad76ffcca0d92c3885c279c80701c2f4a3f3f177?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21691/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21691?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21691      +/-   ##
==========================================
- Coverage    91.9%    91.9%   -0.01%     
==========================================
  Files         154      154              
  Lines       49659    49652       -7     
==========================================
- Hits        45640    45633       -7     
  Misses       4019     4019
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.28% <100%> (-0.01%)` | :arrow_down: |
| #single | `41.9% <15.38%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21691?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/21691/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97.1% <100%> (-0.02%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21691?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/21691?src=pr&el=footer). Last update [ad76ffc...3e81f57](https://codecov.io/gh/pandas-dev/pandas/pull/21691?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @jbrockmendel 