1. Sync sphinx dependency with global requirements. It caps python 2 since
sphinx 2.0 no longer supports Python 2.7.
2. Update some URLs to latest
3. Update the UPPER_CONSTRAINTS_FILE for tox[1]
[1]: refer to http://lists.openstack.org/pipermail/openstack-discuss/2019-May/006478.html
Change-Id: Id095cbd60f5a11f25a64f37b18393afbd4b4262c