TexManager has a complex caching machinery (... among other caching
layers) to map font-related rcParams to a tex preamble (see_rc_cache,
_rc_cache_keys,_reinit) but that's just a matter of a few dict
lookups which are negligible compared to invoking the subprocess; so
just strip out that caching and always regenerate the font-related
preamble.

That caching also set some attributes (texmanager.serif, etc.) as a
side-effect viasetattr/getattr, which are used nowhere else (and
it's hard to know they even exist other than figuring out the relevant
setattr calls); just deprecate them.

(I have some followup cleanups on that...)

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code isFlake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way