Deprecate axis_artist.BezierPath.

[anntzer]

Throughout the library the advice to draw an arbitrary Path object is to
use PathPatch; there's no reason to do differently with a one-off class
in axisartist.

Technically that's an API break because the type of AxisArtist.line
changed, but most common APIs do overlap (e.g. set_color, which is
exercised by the tests) so I'm not going to bother with a deprecation
period which would be quite painful (likely involving a global rcParam
controlling the switch from the old API to the new one).

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