tox: Always recreate docs

The reno extension doesn't seem to trigger an environment rebuild. Force
this.

Signed-off-by: Stephen Finucane <[email protected]>

Author: stephenfin

tox.ini

@@ -51,7 +51,7 @@ exclude = ./patchwork/migrations
 deps =
     -r{toxinidir}/docs/requirements.txt
 commands =
-    sphinx-build -W -b html docs docs/_build/html
+    sphinx-build -E -W -b html docs docs/_build/html
 
 [testenv:lint]
 basepython = python2.7