Workaround for new pip 20.3 behavior
Workaround for new pip 20.3 behavior

This WA set specific pip version to avoid the issue:
"ERROR: Links are not allowed as constraints" [1]
A proper fix would be to adapt to new pip behavior.
Similar issue on Devstack [2][3]

[1] https://github.com/pypa/pip/issues/8210
[2] https://bugs.launchpad.net/devstack/+bug/1906322
[3] https://review.opendev.org/c/openstack/devstack/+/764803

Change-Id: I49d436b0cf1503b148a04f82e1ee41941ebd9d0a

openstack/monasca-api,openstack/monasca-api,stackforge/monasca-api,stackforge/monasca-api,stackforge/monasca-api,openstack/monasca-api,stackforge/monasca-api