Cleanup Typing in pandas.core.strings
- [x] closes #25902 
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25904?src=pr&el=h1) Report
> Merging [#25904](https://codecov.io/gh/pandas-dev/pandas/pull/25904?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/882961df6025b800b899d351c243412e799accf4?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25904/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25904?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25904      +/-   ##
==========================================
+ Coverage   91.53%   91.53%   +<.01%     
==========================================
  Files         175      175              
  Lines       52808    52810       +2     
==========================================
+ Hits        48338    48340       +2     
  Misses       4470     4470
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.1% <100%> (ø)` | :arrow_up: |
| #single | `41.83% <90%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25904?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/25904/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.59% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25904?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/25904?src=pr&el=footer). Last update [882961d...2dddb6e](https://codecov.io/gh/pandas-dev/pandas/pull/25904?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/25904?src=pr&el=h1) Report
> Merging [#25904](https://codecov.io/gh/pandas-dev/pandas/pull/25904?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cb89bc0c012a3154fae394a7299e72ec75956c9b?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/25904/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25904?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25904      +/-   ##
==========================================
- Coverage    91.8%    91.8%   -0.01%     
==========================================
  Files         174      174              
  Lines       52548    52550       +2     
==========================================
- Hits        48243    48242       -1     
- Misses       4305     4308       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.35% <100%> (ø)` | :arrow_up: |
| #single | `41.87% <90%> (-0.08%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25904?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/25904/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.59% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/25904/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `75% <0%> (-12.5%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/25904/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.79% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25904/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.73% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25904?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/25904?src=pr&el=footer). Last update [cb89bc0...33c2278](https://codecov.io/gh/pandas-dev/pandas/pull/25904?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback ping on this one - any other changes needed? thanks @WillAyd nope this one is good