CLN: miscellaneous cleanups / fixes
#21365 made good progress on #21336 so I copied the test from there and referenced it in the whatsnew.

closes #19700
closes #19011
closes #21336
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21861?src=pr&el=h1) Report
> Merging [#21861](https://codecov.io/gh/pandas-dev/pandas/pull/21861?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5d0daa0522730a6f999ceaf328f63f03dd62d0b4?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/21861/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/21861?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21861      +/-   ##
==========================================
- Coverage   91.92%   91.91%   -0.01%     
==========================================
  Files         160      160              
  Lines       49906    49909       +3     
==========================================
+ Hits        45875    45876       +1     
- Misses       4031     4033       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.11% <83.33%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21861?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/inference.py](https://codecov.io/gh/pandas-dev/pandas/pull/21861/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2luZmVyZW5jZS5weQ==) | `98.41% <ø> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/21861/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.11% <100%> (-0.09%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/21861/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.21% <100%> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/21861/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.19% <100%> (ø)` | :arrow_up: |
| [pandas/core/tools/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/21861/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdG9vbHMvdGltZWRlbHRhcy5weQ==) | `98.41% <100%> (ø)` | :arrow_up: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/21861/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `96.94% <0%> (-0.24%)` | :arrow_down: |
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/pull/21861/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `95.42% <0%> (-0.04%)` | :arrow_down: |
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/21861/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `97.05% <0%> (+0.22%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21861?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/21861?src=pr&el=footer). Last update [5d0daa0...84e9663](https://codecov.io/gh/pandas-dev/pandas/pull/21861?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @jbrockmendel keep em coming!