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: Ia888ea1358c4132f7f154ab616b93247f8051d31

openstack/python-monascaclient,stackforge/python-monascaclient,openstack/python-monascaclient,stackforge/python-monascaclient