• Generate a message saying 'The foo attribute has been deprecated'
  instead of 'The foo function'.
• Automatically deprecate the setter at the same time as the getter.
• Also emit the warning when accessing the attribute on the class rather
  than on an instance.

(Compare with the recently removed attempt at supporting deprecation of
classmethods, which didn't really bring anything except the ability to
put the @deprecated decorator over the @classmethod decorator.)

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