Fix docs job

Sphinx 4.0.0 added a new dependency [0] which is causing the job to fail
at the moment.

This patch fix the problem by adding UC to the docs jobs.

[0] https://www.sphinx-doc.org/en/master/changes.html (LaTeX: add
tex-gyre font dependency)

Change-Id: I28019331017405c06577ada88f8e9f6d9a2afc23
Signed-off-by: Lucas Alvares Gomes <lucasagomes@gmail.com>

tox.ini

@@ -34,7 +34,9 @@ commands = bash -c "find {toxinidir}             \
          -print0 | xargs -0 bashate -v -iE006 -eE005,E042"
 
 [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
 whitelist_externals = bash
 setenv =
   TOP_DIR={toxinidir}