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
Depend-On: https://review.opendev.org/#/c/663060/

Change-Id: I8a96eea9d3d8f6d8beae9d86f307f0edd075392c

openstack/python-glanceclient,openstack/python-glanceclient