Special-case usetex minus to zero depth. - #16366

This avoids vertical misalignment between e.g. "$1$" and "$-1$":

for x in [-1, 1]: figtext(.5, .5, f"${x}$", usetex=True, ha="right")

old (note the misalignment of baselines of "1"):

new:

Closes #6323, although more robust (and more complex...) solutions are discussed in #6323 (comment).

Mostly the same as #6333 which this supersedes, but here I explicitly only override the metrics for computer modern symbols (I have no idea whether other fonts have this problem), and added a test.

PR Summary

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