Update sphinx conf.py for > 3.0.0

[crwilcox]

Update templates to incorporate change seen at googleapis/python-bigtable#20

https://github.com/googleapis/synthtool/blob/master/synthtool/gcp/templates/python_library/docs/conf.py.j2
https://github.com/googleapis/synthtool/blob/master/synthtool/gcp/templates/python_library/noxfile.py.j2

[crwilcox]

@kolea2

[SurferJeffAtGoogle]

I don't have enough context to understand the problem alluded to in this bug description.

[crwilcox]

We were forced to pin sphinx due to a breaking change in 3.0.0+. This change has already been made in Bigtable, but other libraries will likely want the same change.

[SurferJeffAtGoogle]

I see, so this bug is really a for-all-the-python-libraries bug? It seems orthogonal to synthtool.