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: I5e798ecc8ee9febfacd7880504ab8f17471fb064

stackforge/monasca-api,stackforge/monasca-api,openstack/monasca-api,openstack/monasca-api,stackforge/monasca-api,openstack/monasca-api,stackforge/monasca-api