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: Iaaa5cae1902bbadae34b555e20d85bbef4bf92b1

tox.ini

@@ -5,7 +5,7 @@ envlist = py27,py37,pep8
 [testenv]
 install_command = pip install {opts} {packages}
 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/train}
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
 # We want to support both vanilla stdlib and eventlet monkey patched