 CLN: pd.TimeGrouper
- [x] xref #6581
- [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/26477?src=pr&el=h1) Report
> Merging [#26477](https://codecov.io/gh/pandas-dev/pandas/pull/26477?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f5cc0789ae5f5a9e5b251e94784f537f30e0b490?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26477/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26477?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26477      +/-   ##
==========================================
- Coverage   91.75%   91.74%   -0.01%     
==========================================
  Files         174      174              
  Lines       50765    50759       -6     
==========================================
- Hits        46578    46568      -10     
- Misses       4187     4191       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.25% <ø> (-0.01%)` | :arrow_down: |
| #single | `41.72% <ø> (-0.09%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26477?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/26477/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXBpLnB5) | `100% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26477/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/26477/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.02% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26477?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/26477?src=pr&el=footer). Last update [f5cc078...caedf0d](https://codecov.io/gh/pandas-dev/pandas/pull/26477?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/26477?src=pr&el=h1) Report
> Merging [#26477](https://codecov.io/gh/pandas-dev/pandas/pull/26477?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f5cc0789ae5f5a9e5b251e94784f537f30e0b490?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26477/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26477?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26477      +/-   ##
==========================================
+ Coverage   91.75%   91.75%   +<.01%     
==========================================
  Files         174      174              
  Lines       50765    50673      -92     
==========================================
- Hits        46578    46494      -84     
+ Misses       4187     4179       -8
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.26% <ø> (ø)` | :arrow_up: |
| #single | `41.7% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26477?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/26477/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXBpLnB5) | `100% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26477/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/dtypes/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/26477/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbW1vbi5weQ==) | `95.85% <0%> (-1.6%)` | :arrow_down: |
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/26477/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `90.96% <0%> (-0.63%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/26477/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `96.36% <0%> (-0.49%)` | :arrow_down: |
| [pandas/core/indexes/frozen.py](https://codecov.io/gh/pandas-dev/pandas/pull/26477/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9mcm96ZW4ucHk=) | `91.78% <0%> (-0.33%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26477/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97% <0%> (-0.14%)` | :arrow_down: |
| [pandas/core/sparse/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/26477/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL3Nlcmllcy5weQ==) | `93.18% <0%> (-0.13%)` | :arrow_down: |
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/26477/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `83.77% <0%> (-0.13%)` | :arrow_down: |
| [pandas/core/internals/managers.py](https://codecov.io/gh/pandas-dev/pandas/pull/26477/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL21hbmFnZXJzLnB5) | `93.87% <0%> (-0.06%)` | :arrow_down: |
| ... and [14 more](https://codecov.io/gh/pandas-dev/pandas/pull/26477/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26477?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/26477?src=pr&el=footer). Last update [f5cc078...d62a80f](https://codecov.io/gh/pandas-dev/pandas/pull/26477?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @mroeschke 