... or rather, add a mention in the docs so that we can link to it when
reviewing PRs.

Given that the section about pyflakes got removed, also remove the
remaining mention about coverage in that list (in fact, it should be
pytest-cov instead; that part really belongs in testing.rst anyways; and
we don't really enforce the "at least 70% rule"...).

(See https://github.com/matplotlib/matplotlib/pull/20242/files#r633145582 (thanks @Illviljan for pointing out the problems with math expressions) and #18796 (comment).)

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).