MNT: move ArtistList into artist module by rcomer · Pull Request #31794 · matplotlib/matplotlib

Notes on second commit for second reviewer:

For some reason, modifying the artist_api.rst triggered warnings and therefore a Circle-CI failure like this. I demonstrated that removing the inheritance diagram at the top of the file made the problem go away, so that seems to be the cause but I'm unclear why the inheritance diagram has not been a problem before now.

To fix it, I went ahead and added docs for the missing private artists.

For _ImageBase and _AnnotationBase I think these are reasonable to include as they have useful methods that do not show in the docs for the public arists. E.g. get_shape, get_size, set_data on _ImageBase and get/set_annotation_clip on _AnnotationBase.

For _AxesBase I made it as unobtrusive as possible.