DOC: Test doctests in pandas/core/groupby/groupby.py
- [x] closes #26254
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26255?src=pr&el=h1) Report
> Merging [#26255](https://codecov.io/gh/pandas-dev/pandas/pull/26255?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7eff62725a3988d6d4ecc9edb87441355e33d75a?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/26255/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26255?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26255      +/-   ##
==========================================
- Coverage   91.98%   91.97%   -0.01%     
==========================================
  Files         175      175              
  Lines       52384    52384              
==========================================
- Hits        48184    48179       -5     
- Misses       4200     4205       +5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.52% <ø> (ø)` | :arrow_up: |
| #single | `40.72% <ø> (-0.14%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26255?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/26255/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGJ5LnB5) | `97.23% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26255/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/26255/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/26255/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/26255?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/26255?src=pr&el=footer). Last update [7eff627...38c2269](https://codecov.io/gh/pandas-dev/pandas/pull/26255?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/26255?src=pr&el=h1) Report
> Merging [#26255](https://codecov.io/gh/pandas-dev/pandas/pull/26255?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7eff62725a3988d6d4ecc9edb87441355e33d75a?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26255/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26255?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26255      +/-   ##
==========================================
- Coverage   91.98%   91.97%   -0.02%     
==========================================
  Files         175      175              
  Lines       52384    52386       +2     
==========================================
- Hits        48184    48180       -4     
- Misses       4200     4206       +6
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.52% <ø> (-0.01%)` | :arrow_down: |
| #single | `40.72% <ø> (-0.14%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26255?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/26255/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGJ5LnB5) | `97.23% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26255/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/26255/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `97.98% <0%> (-0.22%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26255/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/26255/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/26255?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/26255?src=pr&el=footer). Last update [7eff627...9b2dd46](https://codecov.io/gh/pandas-dev/pandas/pull/26255?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 this is probably a good fix, but we are not testing this currently, you need update in ci/code_checks (and see if this passes)

```
    MSG='Doctests generic.py' ; echo $MSG
    pytest -q --doctest-modules pandas/core/generic.py \
        -k"-_set_axis_name -_xs -describe -droplevel -groupby -interpolate -pct_change -pipe -reindex -reindex_axis -to_json -transpose -values -xs -to_clipboard"
    RET=$(($RET + $?)) ; echo $MSG "DONE"
``` Thanks for your guidance. thanks for the patch @HyukjinKwon keep em coming!