offsets/liboffsets cleanup
working on making DateOffset immutable enough unrelated changes added up to merit separating them out into this PR.  No logic is affected.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19922?src=pr&el=h1) Report
> Merging [#19922](https://codecov.io/gh/pandas-dev/pandas/pull/19922?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7561eb8a2a20e7e896cf82a5b8796ae93ccd5574?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19922/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/19922?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19922      +/-   ##
==========================================
+ Coverage   91.66%   91.69%   +0.02%     
==========================================
  Files         150      150              
  Lines       48974    48921      -53     
==========================================
- Hits        44894    44857      -37     
+ Misses       4080     4064      -16
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.06% <100%> (+0.02%)` | :arrow_up: |
| #single | `41.85% <36.36%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19922?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/19922/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97.21% <100%> (+0.2%)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19922/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.95% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19922?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/19922?src=pr&el=footer). Last update [7561eb8...d1f4d3d](https://codecov.io/gh/pandas-dev/pandas/pull/19922?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
