Raise sphinx>=1.7.3 in docs requirements
Raise sphinx>=1.7.3 in docs requirements

To avoid a sphinx bug that ignores exclude_patterns in some
lower versions.
Sphinx bug: https://github.com/sphinx-doc/sphinx/issues/4771

FFE: http://lists.openstack.org/pipermail/openstack-dev/2018-August/133065.html

Change-Id: I34aca28366e5c183ceb01b37d63688ac25332740
Closes-Bug: 1778171

openstack/congress,openstack/congress