The naming (without underscore) is similar to findfont.

I chose to provide an API adding a single path rather than also wrap
findSystemFonts given that the later is basically just a recursive glob
(Path(...).glob("**/*.ttf")).

addfont has no deduplication logic but I think that's fine (really
ttflist should be ttfset, but that's a bigger API change).

findSystemFonts already assumes that the font extension gives the font
type so assuming the same in addfont seems fine.

See discussion starting at https://gitter.im/matplotlib/matplotlib?at=5d54195ea4efe3718dff8b64, attn @andrzejnovak @tacaswell

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