Add upper constraints

py27 is currently broken due to a Sphinx failure that is resolved if you
properly include the upper constraints.

Change-Id: I4bcafb92bd307fe469bcab4db760d4245ceecb76

tox.ini

@@ -6,7 +6,9 @@ envlist = pep8,py37,py36,py35,py27
 [testenv]
 usedevelop = True
 install_command = pip install {opts} {packages}
-deps = -r{toxinidir}/requirements.txt
+deps =
+       -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands=
     python setup.py testr --slowest --testr-args='{posargs}'