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 wallaby instead of
master - https://review.opendev.org/c/openstack/tempest/+/783319

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: I3e5202076c0d318e41218af3ea181a2f2985abec
1 file changed