Blacklist sphinx 2.1.0 (autodoc bug)
Blacklist sphinx 2.1.0 (autodoc bug)

See https://github.com/sphinx-doc/sphinx/issues/6440 for upstream details

Change-Id: Id834a962ea869aa0d60295fbb6d45e0895400e96

openstack/monasca-notification,openstack/monasca-notification