Reduce test requirements duplication, sync flake8 and related (#28538)

* Generate pre-commit test dependencies instead of duplicating

* Upgrade/sync to flake8 3.7.9, flake8-docstrings 1.5.0, and pydocstyle 4.0.1

https://flake8.readthedocs.io/en/latest/release-notes/3.7.9.html
https://gitlab.com/pycqa/flake8-docstrings/blob/1.4.0/HISTORY.rst
https://gitlab.com/pycqa/flake8-docstrings/blob/1.5.0/HISTORY.rst
http://www.pydocstyle.org/en/4.0.1/release_notes.html

* Include requirements_test.txt from *_all.txt instead of copying