Fix upper-constraints.txt url

Wrong link used in commit 203bce9cd7.

Change-Id: I90cf7584830afccdd0028b0ed7e5061e67430815

tox.ini

@@ -8,7 +8,7 @@ setenv = VIRTUAL_ENV={envdir}
          PYTHONWARNINGS=default::DeprecationWarning
          OS_TEST_PATH=heat/tests
 usedevelop = True
-install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/src/branch/master/upper-constraints.txt} {opts} {packages}
+install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =