Hmm, I'm not sure we can document the nested class, as that seems to be what trips Sphinx up:

WARNING: autodoc: failed to import class 'ArtistList' from module 'Axes'; the following exception was raised:
No module named 'Axes'

Ok, I have replaced the autosummary entry with a separate autoclass call, which can handle inner classes. This is not quite is nice because it dums the whole class docs instead of a summary with a link. But ArtistList is only a temporary helper structure, which will vanish again in 3.7. So it's not worth fiddling too much with sphinx. The main point that the user can find it and read what it is is fulfilled.

You'll need to rebase to get #22190 for docs to build.