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: Ia4ad99c8201f48292c35e7b50604d1d87c467eb3

openstack/tacker,openstack/tacker,openstack/tacker,stackforge/tacker,stackforge/tacker