PR Summary

Following up the comment: #20266 (comment), this PR adds a override_math to Text objects, and make TextBox's text object default it to True.

This resolves #20266.
It does not allow GUI to go in invalid state when there's invalid mathtext in it, because it isn't parsed at all.
(also allows folks to override mathtext at all in their text objects)

Interactive trial:

import matplotlib.pyplot as plt
from matplotlib.widgets import TextBox

fig, ax = plt.subplots()
fig.subplots_adjust(left=0.25, top=0.55, bottom=0.45)
text_box = TextBox(ax, "Something in '\$ \$'")                       # try typing wrong mathtext here

plt.show()

Static trial:

import matplotlib.pyplot as plt
fig, ax = plt.subplots()
ax.text(1, 2, "$$")
plt.show()

Needs a changenote (API change) and tests, but let's wait for a review before that.

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).