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: I5b339d69038db95dc339315511a1e4e949b38cb2

tox.ini

@@ -7,7 +7,7 @@ envlist = pep8,py27,py37
 usedevelop = True
 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}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = python setup.py testr --slowest --testr-args='{posargs}'