Upgrade flake8 and dependencies, enable flake8-noqa (#48393)
* Upgrade flake8 to 3.9.0
https://flake8.pycqa.org/en/latest/release-notes/3.9.0.html
* Upgrade pydocstyle to 6.0.0
https://www.pydocstyle.org/en/latest/release_notes.html#september-13th-2020
https://www.pydocstyle.org/en/latest/release_notes.html#march-18th-2021
* Upgrade flake8-docstrings to 1.6.0, enable flake8-noqa
https://gitlab.com/pycqa/flake8-docstrings/-/blob/1.6.0/HISTORY.rst
https://github.com/plinss/flake8-noqa/issues/1
* Upgrade/pin pyflakes to 2.3.1
https://github.com/PyCQA/pyflakes/blob/2.3.1/NEWS.rst
* Pin pycodestyle to 2.7.0
3cd52b695d