Use upper-constraints in docs builds
We've just got hit by cffi pypi package upgrade. This is because we're
not using upper-constraints in our `docs` tox environment. This commit
fixes that by adding it to the env.
@@ -52,7 +52,8 @@ commands =
[testenv:docs]
basepython = python3
-deps = -r{toxinidir}/doc/requirements.txt
+deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+ -r{toxinidir}/doc/requirements.txt
commands = sphinx-build -W -b html doc/source doc/build/html
[flake8]
