See #23381 (comment) for motivation. Supersedes #23381. @painch Can you check whether this works for you?

Directly support ^% in pgf.

Support for ^ is inspired from the support for _ as implemented
in underscore.sty, but much simpler because 1) we don't care about
hyphenation, 2) we don't care about substituting a real underscore by
a rule box when the current font lacks an underscore (there's no real
way out if the font has no caret, anyways), and 3) we don't care about
the active character ending up in tex \commands because the character
is only made active in the context of matplotlib-provided strings, not
globally.

Also handle \displaystyle and \mathdefault at the TeX level.

Note that tex_escape is still being tested by
test_minus_signs_with_tex, which explicitly checks support for unicode
minus.

Edit: A test fails on the Ubuntu 18.04 texlive (and I can repro this locally) with pdflatex only because the strings get shifted a tiny bit. I'm actually a bit at loss as to why this shift occurs, and especially only with pdflatex on an older texlive. (In particular, I have separately checked the reported text metrics, which did not change; more debugging indicates that it's the \everymath{\displaystyle} which seems to very slightly shift the non-math text.) I would be tempted to just skip this test on the old Ubuntu 18.04 texlive (considering that it's just a slight image shift, and that the test still passes on the more recent Ubuntu image...
Edit^2: Looks like I can just get away with bumping the test tolerance by 1-per-thousand on old ghostscripts...

PR Summary

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• New features are documented, with examples if plot related.
• 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).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).