Fix PEP-8 issues in contributing.rst
```
doc/source/contributing.rst:675:5: E302 expected 2 blank lines, found 1
doc/source/contributing.rst:684:5: E302 expected 2 blank lines, found 1
```
Requires #23847 before merging.
 @FHaase, I think you can merge master and leave the CI in green now.

Also, if you can remove the file from `setup.cfg` # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24003?src=pr&el=h1) Report
> Merging [#24003](https://codecov.io/gh/pandas-dev/pandas/pull/24003?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/92d25f0da6c3b1175047cba8c900e04da68920b8?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24003/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24003?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24003   +/-   ##
=======================================
  Coverage   42.45%   42.45%           
=======================================
  Files         161      161           
  Lines       51561    51561           
=======================================
  Hits        21888    21888           
  Misses      29673    29673
```

| Flag | Coverage Δ | |
|---|---|---|
| #single | `42.45% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24003?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/24003/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `33% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24003?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/24003?src=pr&el=footer). Last update [92d25f0...7d419e5](https://codecov.io/gh/pandas-dev/pandas/pull/24003?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/24003?src=pr&el=h1) Report
> Merging [#24003](https://codecov.io/gh/pandas-dev/pandas/pull/24003?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/92d25f0da6c3b1175047cba8c900e04da68920b8?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24003/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24003?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24003   +/-   ##
=======================================
  Coverage   42.45%   42.45%           
=======================================
  Files         161      161           
  Lines       51561    51561           
=======================================
  Hits        21888    21888           
  Misses      29673    29673
```

| Flag | Coverage Δ | |
|---|---|---|
| #single | `42.45% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24003?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/24003/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `33% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24003?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/24003?src=pr&el=footer). Last update [92d25f0...7d419e5](https://codecov.io/gh/pandas-dev/pandas/pull/24003?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @FHaase 