Scanpy: Better CI for docs

[ivirshup]

@flying-sheep @falexwolf

Doc builds are broken ... again. This really has to be part of the CI process. It should be implemented in a way where the CI marks a PR as failing if read the docs would error.

I see two options:

• Get the readthedocs CI builds (which we can ask them about)
• Figure out how to make our current travis builds error like read the docs does.

@giovp I don't believe you can mark keys within a dictionary as attributes. I think that's causing this error:

Warning, treated as error:
/home/docs/checkouts/readthedocs.org/user_builds/icb-scanpy/checkouts/latest/scanpy/readwrite.py:docstring of scanpy.read_visium:38:py:attr reference target not found: anndata.AnnData.uns['spatial']['library_id']

[giovp]

Sorry for that @ivirshup ! I saw you already fixed it in #1145 , thank you and I will keep it in mind for next time.

[ivirshup]

We've got CI doc builds now!