[cliff][docs][requirements] new cliff versions causes docs to fail to build

Looks like some things changed in the new version that we depended upon and are now causing failures. Exception occurred: File "/home/zuul/src/opendev.org/openstack/python-openstackclient/.tox/docs/lib/python3.6/site-packages/cliff/sphinxext.py", line 245, in _load_app if not issubclass(cliff_app_class, app.App): TypeError: issubclass() arg 1 must be a class -- Matthew Thode