DOC: html build of class docstrings without autosummary shows None
We have some hacks to have class docstring pages without needing to build pages for all its methods/attributes.

For 0.22 docs this was working nicely:

https://pandas.pydata.org/pandas-docs/version/0.22.0/generated/pandas.Int64Index.html

But with 0.23, this now lists 'None' (which is a bit ugly on the page, but not a critical issue):

https://pandas.pydata.org/pandas-docs/version/0.23.0/generated/pandas.Int64Index.html

So we should investigate if something changes on pandas or sphinx side. 
(the pandas docs sadly don't show the sphinx version it was built with)
