Switch back the tox constraint to master

Tempest 35.0.0 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/+/885090/1

Change-Id: I02559c88825b69477958a6114d93bd428a7759ca
1 file changed