Fix Up Typing in GroupBy
More typing cleanups

Original failures:

```sh
pandas/core/groupby/ops.py:13: error: Module 'pandas._libs' has no attribute 'groupby'
pandas/core/groupby/ops.py:13: error: Module 'pandas._libs' has no attribute 'reduction'
pandas/core/groupby/groupby.py:22: error: Module 'pandas._libs' has no attribute 'groupby'
pandas/core/groupby/groupby.py:329: error: Need type annotation for '_apply_whitelist'
pandas/core/groupby/generic.py:220: error: Incompatible types in assignment (expression has type "Callable[[Arg(Any, 'arg'), VarArg(Any), KwArg(Any)], Any]", base class "SelectionMixin" defined the type as "Callable[[Arg(Any, 'func'), VarArg(Any), KwArg(Any)], Any]")
```
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26089?src=pr&el=h1) Report
> Merging [#26089](https://codecov.io/gh/pandas-dev/pandas/pull/26089?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7b3bf2dc6b8b00133d2cf46c1f70f2865666a13f?src=pr&el=desc) will **decrease** coverage by `51.2%`.
> The diff coverage is `72.72%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26089/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26089?src=pr&el=tree)

```diff
@@             Coverage Diff             @@
##           master   #26089       +/-   ##
===========================================
- Coverage   91.96%   40.75%   -51.21%     
===========================================
  Files         175      175               
  Lines       52412    52447       +35     
===========================================
- Hits        48200    21376    -26824     
- Misses       4212    31071    +26859
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `?` | |
| #single | `40.75% <72.72%> (-0.12%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26089?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/26089/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGJ5LnB5) | `24.59% <100%> (-72.63%)` | :arrow_down: |
| [pandas/core/groupby/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/26089/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9vcHMucHk=) | `20.04% <100%> (-74.17%)` | :arrow_down: |
| [pandas/core/groupby/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/26089/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9nZW5lcmljLnB5) | `13.3% <25%> (-73.79%)` | :arrow_down: |
| [pandas/io/formats/latex.py](https://codecov.io/gh/pandas-dev/pandas/pull/26089/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvbGF0ZXgucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/io/sas/sas\_constants.py](https://codecov.io/gh/pandas-dev/pandas/pull/26089/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNfY29uc3RhbnRzLnB5) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/core/groupby/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/26089/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9jYXRlZ29yaWNhbC5weQ==) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/tseries/plotting.py](https://codecov.io/gh/pandas-dev/pandas/pull/26089/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvcGxvdHRpbmcucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/tseries/converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/26089/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvY29udmVydGVyLnB5) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/io/formats/html.py](https://codecov.io/gh/pandas-dev/pandas/pull/26089/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvaHRtbC5weQ==) | `0% <0%> (-99.37%)` | :arrow_down: |
| [pandas/io/sas/sas7bdat.py](https://codecov.io/gh/pandas-dev/pandas/pull/26089/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXM3YmRhdC5weQ==) | `0% <0%> (-91.16%)` | :arrow_down: |
| ... and [136 more](https://codecov.io/gh/pandas-dev/pandas/pull/26089/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26089?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/26089?src=pr&el=footer). Last update [7b3bf2d...00ea674](https://codecov.io/gh/pandas-dev/pandas/pull/26089?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/26089?src=pr&el=h1) Report
> Merging [#26089](https://codecov.io/gh/pandas-dev/pandas/pull/26089?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8b60ebb3598881571b390276d2c75ec7c7a95212?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/26089/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26089?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26089      +/-   ##
==========================================
- Coverage   91.99%   91.98%   -0.01%     
==========================================
  Files         175      175              
  Lines       52384    52387       +3     
==========================================
- Hits        48189    48187       -2     
- Misses       4195     4200       +5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.53% <100%> (ø)` | :arrow_up: |
| #single | `40.74% <72.72%> (-0.13%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26089?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/26089/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGJ5LnB5) | `97.23% <100%> (ø)` | :arrow_up: |
| [pandas/core/groupby/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/26089/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9nZW5lcmljLnB5) | `89.02% <100%> (ø)` | :arrow_up: |
| [pandas/core/groupby/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/26089/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9vcHMucHk=) | `94.23% <100%> (+0.02%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26089/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/26089/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.9% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26089/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.61% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26089?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/26089?src=pr&el=footer). Last update [8b60ebb...162efaa](https://codecov.io/gh/pandas-dev/pandas/pull/26089?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @WillAyd 