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: I6cf3c51168ea0571501a01ca44ea2134ec281f68

openstack/blazar,openstack/blazar,ChameleonCloud/blazar,stackforge/blazar,ChameleonCloud/blazar,stackforge/blazar