PR Summary

Text.is_math_text is only ever called with self.get_usetex() as second
argument, so just change it to a private method (Text._preprocess_math)
that calls self.get_usetex() internally; deprecate is_math_text which
is clearly a private internal helper. This also helps clarifying that
usetex mode is only ever called if self.get_usetex() is True (in the
textobj.is_math_text(...) line, textobj is actually the same object(!)
as self so shares the same get_usetex() value).
(#13166 also clarifies the situation with get_usetex()).

In Text._get_layout, if self.get_usetex() is True (and only in that
case), self._fontproperties can refer to a TeX-only font (e.g. "Computer
Modern Roman"). If that's the case, don't try to call
renderer.get_text_width_height_descent with ismath=False, as that would
trigger a font resolution that can fail (perhaps there's no "Computer
Modern Roman" font available in the font cache; it may only be available
to TeX) and cause a spurious warning.

Goes on top of #12951 to avoid a rebase.
Closes (probably) #13139, at least #13139 (comment).

PR Checklist

• Has Pytest style unit tests
• Code is Flake 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