1. Sync sphinx dependency with global requirements. It caps python 2 since
sphinx 2.0 no longer supports Python 2.7.
2. Update the UPPER_CONSTRAINTS_FILE for tox[1]
3. Update some URLs to latest
[1]: http://lists.openstack.org/pipermail/openstack-discuss/2019-May/006478.html
Change-Id: Ibe8c402c2817c31748e6a4b8344024a276609260