update sphinx requirement
update sphinx requirement

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

Change-Id: Ic909ffca550329c8ba23e4ffe9f18f601d674a3b

openstack/python-tackerclient,stackforge/python-tackerclient,openstack/python-tackerclient,stackforge/python-tackerclient