Switch back the tox constraint to master

Tempest 44.0.0 release hash is prepared[1] so we can
revert back to use the master constraints by default.

[1] https://review.opendev.org/c/openstack/releases/+/952405

Change-Id: Id2914f27d0a5909c599803d8b4a948cab02c665c
2 files changed