Switch to the master constraint in tox.ini

To releas a new tag for Tempest, we modified the
compatible constraint in tox.ini
- https://review.opendev.org/c/openstack/tempest/+/882354/

As new tag is released now so we can switch back to the master
constraint in tox.ini.

Change-Id: Ic4f60c6a1b8b657edf06cffc9165c49da4395389
1 file changed