Fix upper constraints

It is incorrectly pulling in master which is leading to a pyflakes
problem.

Change-Id: I1ab4ed3d7caca81b77c28b573e953af2cbaf6824

tox.ini

@@ -9,7 +9,7 @@ install_command = pip install -U {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
 deps =
-       -c{env:UPPER_CONSTRAINTS_FILE: https://releases.openstack.org/constraints/upper/master}
+       -c{env:UPPER_CONSTRAINTS_FILE: https://releases.openstack.org/constraints/upper/train}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = stestr run --slowest {posargs}