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