Switch to the new canonical constraints URL on master

Reference:
1. http://lists.openstack.org/pipermail/openstack-discuss/2019-May/006478.html
2. https://review.opendev.org/#/c/601188

Change-Id: I87a908e1965fe9a0785329533eab7b9ce2ded8cb

tox.ini

@@ -7,8 +7,9 @@ skipsdist = True
 basepython=python3
 usedevelop = True
 setenv = VIRTUAL_ENV={envdir}
-install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt} {opts} {packages}
-deps = -r{toxinidir}/test-requirements.txt
+install_command = pip install {opts} {packages}
+deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -r{toxinidir}/test-requirements.txt
 whitelist_externals = find
 commands =
     find . -type f -name "*.pyc" -delete