Block Sphinx 3.0.0
Block Sphinx 3.0.0

Sphinx 3.0.0 breaks the building here, block it for now.

Depends-On: https://review.opendev.org/#/c/717949/

Change-Id: Ibf0c93ea79fec647fbf749257835f1fa99d5f59d

openstack/watcher,stackforge/watcher,openstack/watcher,stackforge/watcher