Switch to the master constraint in tox.ini

To releas a new tag for Tempest Zed, we modified the
compatible constraint in tox.ini
- https://review.opendev.org/c/openstack/tempest/+/857602/2

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

Change-Id: I20f8d726d5b9adf9e5b7669c1b3ff8b28d46f0ac
1 file changed