... instead of delaying it until rendering time. This makes it
consistent with the behavior of rcParams["font.family"]. (While lazy
resolution may be useful too, it seems better to be consistent with
family, whose behavior is constrained by backcompat, whereas
mathtext_fontfamily has not been released yet.)

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).