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: Id7033122874f1ebbe1b659fae854b21003e5c0e1

openstack/ironic,openstack/ironic