Use the master constraint in tox.ini again

While releasing a new tag for Tempest, we modified the
constraint in tox.ini so that they are set to xena instead of
master - https://review.opendev.org/c/openstack/tempest/+/808031

Now the new tag is released and we can bring back the master
constraint in tox.ini so that Tempest master uses master constraint.

Change-Id: Iaf9eaaf60ea3c2d15b1225f713364ebede25768a
1 file changed