DEP/CI: Pin flake8-rst version

[FHaase]

• closes flake8-rst 0.4.3 config? #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]

Codecov Report

Merging #23699 into master will not change coverage.
The diff coverage is n/a.

@@           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)	⬆️
#single	42.34% <ø> (ø)	⬆️

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[jreback]

thanks @FHaase