Switch back the tox constraint to master

Tempest 37.0.0 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/+/908346

Change-Id: I6487792f9bd6aa6c6383ae6448038f6d77a5beb9
2 files changed