The version in portage (2.2.0-r1) hasn't been updated since last july, while upstream has released several versions. Currently at 2.6.0 if i understand well.
3.0.0 is for mitaka, I'll update it. https://github.com/openstack/requirements/blob/stable/mitaka/upper-constraints.txt#L306 Didn't know anyone used it :P
fixed
Seen, thanks !