PR Summary

Closes #17906

• Added a keyword argument that can specify the fontset of a Text object. (The paremeter is fontset= or mathtext_fontset=)
• The parameter is passed to the FontProperties class, a new variable called self._mathtext_fontset was created to hold this value and the necessary functions to make this work were created.
• A pytest function called "test_mathtext_fontset" was created in "test_mathtext.py"
• Tests are ok.
• Added an example to show the new parameter in action: examples/mathtext_fontset_example.py

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) [Not major feature.]
• Documented in doc/api/next_api_changes/* if API changed in a backward-incompatible way [API didn't change in an incompatible way.]