Switch back the tox constraint to master

Tempest 33.0.0 release hash is prepared[1] so we can
revert back to use the master constraints by default.

[1] https://review.opendev.org/c/openstack/tempest/+/864372/

Change-Id: I1a3cea4cc543aabe61483a32f71b2b9db178cecf
1 file changed