Add gitpython to requirements
- [X] closes #26142
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

why are you adding this?  @jreback the command `python make.py html` was failing because gitpython wasn't installed in requirements. you must have an old version: https://github.com/pandas-dev/pandas/blob/master/requirements-dev.txt @jreback You're right. Sorry about that. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26143?src=pr&el=h1) Report
> Merging [#26143](https://codecov.io/gh/pandas-dev/pandas/pull/26143?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f53bb061939fb54d6c826c4db408c458d3d2a1db?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/26143/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26143?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26143      +/-   ##
==========================================
- Coverage   91.99%   91.98%   -0.01%     
==========================================
  Files         175      175              
  Lines       52383    52383              
==========================================
- Hits        48188    48184       -4     
- Misses       4195     4199       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.54% <ø> (ø)` | :arrow_up: |
| #single | `40.74% <ø> (-0.14%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26143?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26143/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/26143/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.9% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26143?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/26143?src=pr&el=footer). Last update [f53bb06...a422673](https://codecov.io/gh/pandas-dev/pandas/pull/26143?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/26143?src=pr&el=h1) Report
> Merging [#26143](https://codecov.io/gh/pandas-dev/pandas/pull/26143?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f53bb061939fb54d6c826c4db408c458d3d2a1db?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/26143/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26143?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26143      +/-   ##
==========================================
- Coverage   91.99%   91.98%   -0.01%     
==========================================
  Files         175      175              
  Lines       52383    52383              
==========================================
- Hits        48188    48184       -4     
- Misses       4195     4199       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.54% <ø> (ø)` | :arrow_up: |
| #single | `40.74% <ø> (-0.14%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26143?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26143/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/26143/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.9% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26143?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/26143?src=pr&el=footer). Last update [f53bb06...a422673](https://codecov.io/gh/pandas-dev/pandas/pull/26143?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
