This essentially reverts Icd31caa93587614425036fc4c8d3cfd3a83b6b3c,
as the incompatibility with six 1.4.x has been fixed meanwhile.
Change-Id: I40c588ca005ea3a8b04e64cfe3b86bbed69ee0a0
Manual first sync with the global openstack/requirements repo. Moving
forward, these will be auto-proposed by a bot on commit to requirements.
Change-Id: I22d2d6c7fb991556ebc156fa94273a37c3e6d3c6