Update TOX/UPPER_CONSTRAINTS_FILE for stable/ussuri

Update the URL to the upper-constraints file to point to the redirect
rule on releases.openstack.org so that anyone working on this branch
will switch to the correct upper-constraints list automatically when
the requirements repository branches.

Until the requirements repository has as stable/ussuri branch, tests will
continue to use the upper-constraints list on master.

Change-Id: Icb553328f98a2d4ce3bbde756ef3351e901b4deb

tox.ini

@@ -8,7 +8,7 @@ ignore_basepython_conflict = True
 basepython = python3
 usedevelop = True
 whitelist_externals = rm
-install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
+install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/ussuri} {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
    PYTHONWARNINGS=default::DeprecationWarning
@@ -47,7 +47,7 @@ commands = oslo_debug_helper {posargs}
 
 [testenv:osc_plugins]
 deps =
-  -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/ussuri}
   -r{toxinidir}/requirements.txt
   ../../x/pbrx
 whitelist_externals =