Fix PDF docs build

Update tox config to include upper-constraints as dependencies. This
resolves issues with building PDF docs.

Change-Id: I98f287e4654ecb6b92d4f3815f359b205b06559a

Author: priteau

tox.ini

@@ -45,7 +45,9 @@ commands = flake8
 commands = {posargs}
 
 [testenv:docs]
-deps = -r{toxinidir}/doc/requirements.txt
+deps =
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build --keep-going -b html doc/source doc/build/html
 
 [testenv:pdf-docs]