Fix type annotations in pandas.core.resample
Part of #25882
- [X] tests passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

This PR addresses the following mypy errors in **pandas.core.resample**:
```
pandas/core/resample.py:34: error: Need type annotation for '_shared_docs_kwargs'
pandas/core/resample.py:876: error: All conditional function variants must have identical signatures
pandas/core/resample.py:884: error: All conditional function variants must have identical signatures
pandas/core/resample.py:891: error: All conditional function variants must have identical signatures
pandas/core/resample.py:1101: error: Definition of "_upsample" in base class "_GroupByMixin" is incompatible with definition in base class "DatetimeIndexResampler"
pandas/core/resample.py:1101: error: Definition of "_upsample" in base class "_GroupByMixin" is incompatible with definition in base class "Resampler"
pandas/core/resample.py:1101: error: Definition of "_downsample" in base class "_GroupByMixin" is incompatible with definition in base class "DatetimeIndexResampler"
pandas/core/resample.py:1101: error: Definition of "_groupby_and_aggregate" in base class "_GroupByMixin" is incompatible with definition in base class "Resampler"
pandas/core/resample.py:1217: error: Definition of "_upsample" in base class "_GroupByMixin" is incompatible with definition in base class "PeriodIndexResampler"
pandas/core/resample.py:1217: error: Definition of "_upsample" in base class "_GroupByMixin" is incompatible with definition in base class "DatetimeIndexResampler"
pandas/core/resample.py:1217: error: Definition of "_upsample" in base class "_GroupByMixin" is incompatible with definition in base class "Resampler"
pandas/core/resample.py:1217: error: Definition of "_downsample" in base class "_GroupByMixin" is incompatible with definition in base class "PeriodIndexResampler"
pandas/core/resample.py:1217: error: Definition of "_downsample" in base class "_GroupByMixin" is incompatible with definition in base class "DatetimeIndexResampler"
pandas/core/resample.py:1217: error: Definition of "_groupby_and_aggregate" in base class "_GroupByMixin" is incompatible with definition in base class "Resampler"
pandas/core/resample.py:1247: error: Definition of "_upsample" in base class "_GroupByMixin" is incompatible with definition in base class "DatetimeIndexResampler"
pandas/core/resample.py:1247: error: Definition of "_upsample" in base class "_GroupByMixin" is incompatible with definition in base class "Resampler"
pandas/core/resample.py:1247: error: Definition of "_downsample" in base class "_GroupByMixin" is incompatible with definition in base class "DatetimeIndexResampler"
pandas/core/resample.py:1247: error: Definition of "_groupby_and_aggregate" in base class "_GroupByMixin" is incompatible with definition in base class "Resampler"
pandas/core/resample.py:1299: error: Incompatible types in assignment (expression has type "Tuple[str, ...]", base class "Grouper" defined the type as "Tuple[str, str, str, str, str]")
```
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26398?src=pr&el=h1) Report
> Merging [#26398](https://codecov.io/gh/pandas-dev/pandas/pull/26398?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e5d15b20e828c66182a4e175dd5a5b9b951d9950?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/26398/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26398?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26398      +/-   ##
==========================================
- Coverage   91.69%   91.68%   -0.01%     
==========================================
  Files         174      174              
  Lines       50749    50752       +3     
==========================================
- Hits        46534    46532       -2     
- Misses       4215     4220       +5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.19% <100%> (ø)` | :arrow_up: |
| #single | `41.17% <100%> (-0.15%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26398?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/26398/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `97.23% <100%> (ø)` | :arrow_up: |
| [pandas/core/groupby/grouper.py](https://codecov.io/gh/pandas-dev/pandas/pull/26398/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGVyLnB5) | `98.53% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26398/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26398/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.02% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26398/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.6% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26398?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/26398?src=pr&el=footer). Last update [e5d15b2...1abd099](https://codecov.io/gh/pandas-dev/pandas/pull/26398?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/26398?src=pr&el=h1) Report
> Merging [#26398](https://codecov.io/gh/pandas-dev/pandas/pull/26398?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e5d15b20e828c66182a4e175dd5a5b9b951d9950?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/26398/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26398?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26398      +/-   ##
==========================================
- Coverage   91.69%   91.68%   -0.01%     
==========================================
  Files         174      174              
  Lines       50749    50752       +3     
==========================================
- Hits        46534    46532       -2     
- Misses       4215     4220       +5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.19% <100%> (ø)` | :arrow_up: |
| #single | `41.17% <100%> (-0.15%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26398?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/26398/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `97.23% <100%> (ø)` | :arrow_up: |
| [pandas/core/groupby/grouper.py](https://codecov.io/gh/pandas-dev/pandas/pull/26398/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGVyLnB5) | `98.53% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26398/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26398/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.02% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26398/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.6% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26398?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/26398?src=pr&el=footer). Last update [e5d15b2...1abd099](https://codecov.io/gh/pandas-dev/pandas/pull/26398?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks