Switch back the tox constraint to master

Tempest 42.0.0 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/+/933772/1

Change-Id: I5267cc99d1ad6572fc8546d0338f25e7fad01481
2 files changed