Add upper constraints to tox

Sphinx has released a version that has dropped python2 support but this
is handled in upper-constraints if we were properly including it in the
tox.ini deps.

Change-Id: I3d0a91ccef9693fd46e19292f4bb21c1ec9a9a19

tox.ini

@@ -8,7 +8,9 @@ usedevelop = True
 install_command = pip install -U {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
-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 = stestr run --slowest {posargs}