This includes changes from:

astropy/astropy@e98e8c3
astropy/astropy#2292 (smart resolver)
astropy/astropy#2376 (autoclass_content=both)
astropy/astropy#2370
astropy/astropy#2430

cc @embray, can you check whether this undoes any of the Py2 -> Py3 changes you made? I think we should merge this before making any more Py2 -> Py3 changes.

Merging so as to get things working in the core package again. We can worry about making Sphinx Python-3-compatible later.

This was going to be my next step, but thanks for doing it for me. I'm puzzled though that before we merged this that the astropy docs were building just fine as far as I could tell.

@embray - actually the RTD builds are broken since we use astropy-helpers, because of this issue: astropy/astropy#2532 - RTD uses make html.