DEP/CI: Pin flake8-rst version
- [x] closes #23665

Pins version of `flake8-rst` to `0.4.2`. 

* Newer versions of this plugin will cover more code-blocks within the documentation. Thus CI will fail when new version is released to conda-forge. 
* Devs using pip already get issues as version `0.4.3` is released to PyPI.

`flake8-rst` is getting upgraded shortly to a version `>0.4.3`. I'm currently updating the doc so it becomes compliant with that newer version, therefore I don't open issues to fix the doc for version `0.4.3`. 

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23699?src=pr&el=h1) Report
> Merging [#23699](https://codecov.io/gh/pandas-dev/pandas/pull/23699?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/fb4405d5ebc132f50103b500592109988ad6a0c5?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/23699/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23699?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23699   +/-   ##
=======================================
  Coverage   92.24%   92.24%           
=======================================
  Files         161      161           
  Lines       51340    51340           
=======================================
  Hits        47361    47361           
  Misses       3979     3979
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.64% <ø> (ø)` | :arrow_up: |
| #single | `42.34% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23699?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/23699?src=pr&el=footer). Last update [fb4405d...017f0f8](https://codecov.io/gh/pandas-dev/pandas/pull/23699?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @FHaase 