Update sphinx requirements
Update sphinx requirements

Following recent changes to global requirements [1], we need
to update sphinx requirements locally. This should fix the
requirements-check job.

[1] If558f184c959e4b63b56dec3ca1571d1034cfe5c

Change-Id: I641a28790bf5df0cec09b5600e2fd83a513dd896

openstack/blazar,ChameleonCloud/blazar,ChameleonCloud/blazar,stackforge/blazar,openstack/blazar,stackforge/blazar