Update sphinx dependency
Update sphinx dependency

Sphinx 2.0 no longer works on python 2.7, so we need to start capping
it there as well
depend on https://review.opendev.org/#/c/657890/

Change-Id: I87d71ce47e5aed5edbb2f8f9a7a0f6439255278e

openstack/mistral-dashboard,openstack/mistral-dashboard,openstack/mistral-dashboard