install only the minimum required to build our docs for rtd (#4133)

* install only the minimum required to build our docs for rtd

We don't need to install sphinxcontrib-spelling or doc8 in rtd.

* move another package
diff --git a/tox.ini b/tox.ini
index 03c4c07..6dfe1c7 100644
--- a/tox.ini
+++ b/tox.ini
@@ -37,6 +37,7 @@
 
 [testenv:docs]
 extras =
+    docs
     docstest
 basepython = python2.7
 commands =