Update TOX/UPPER_CONSTRAINTS_FILE for stable/train

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/train branch, tests will
continue to use the upper-constraints list on master.

Change-Id: I831ad5f5373f6e78a35ed1aa7a7833465c2b4304

tox.ini

@@ -8,7 +8,7 @@ setenv = VIRTUAL_ENV={envdir}
          PYTHONWARNINGS=default::DeprecationWarning
 passenv = TRACE_FAILONLY GENERATE_HASHES http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 usedevelop = True
-deps = -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/stable/stein/upper-constraints.txt}
+deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/train}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 whitelist_externals =