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: I8d0760ce4c44cafb004deb88b15185f140b9cb5d

tox.ini

@@ -5,11 +5,13 @@ skipsdist = True
 
 [testenv]
 usedevelop = True
-install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt} {opts} {packages}
+install_command = pip install {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
    PYTHONWARNINGS=default::DeprecationWarning
-deps = -r{toxinidir}/test-requirements.txt
+deps =
+    -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+    -r{toxinidir}/test-requirements.txt
 commands = python setup.py test --slowest --testr-args='{posargs}'
 
 [testenv:pep8]