Updated openstack/openstack
Project: openstack/oslo.concurrency  73394c2a923b5ece78c39515b02d6974d2b52241

Fix coverage testing

Add coverage to test-requirements so that tox-e cover works - this
is part of the post jobs that are run.

Change-Id: Ie8e5cb47068baa9c05da43efca718222ab841e0a