• validate_bool_maybe_none is now unused; if we really need it back
  we'd make it private and I'd anyways rather add machinery so that
  one can just specify it as [bool, None] (or Optional[bool]) in
  _prop_validators.
• validate_hinting will become a plain list after the deprecation
  period.
• validate_movie_writer makes it impossible to set movie.writer to
  a third-party animation writer, because that third-party module
  will necessarily first import matplotlib before being able to
  register its writer, and thus the matplotlibrc file will be parsed
  before the writer is registered. In any case,
  MovieWriterRegistry.__getitem__ also raises an informative error
  on unknown writer names, which should be enough, so let's just
  accept any string.

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