Sync Sphinx requirement
Sync Sphinx requirement

Sync sphinx dependency with global requirements. It caps python 2 since
sphinx 2.0 no longer supports Python 2.7.

Change-Id: I1a4715a60403ed9a76c9f2c55e566d83395bad7e
Closes-Bug: 1831533

openstack/tosca-parser,openstack/tosca-parser