Add Sphinx to test-requires.

Needed for the gate-zuul-docs job.

Change-Id: Ib330aeaf322543ab1f8b92e48672b0b0b4a4f692
Reviewed-on: https://review.openstack.org/10681
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins
1 file changed