#4290: remove skip_install in testenv docs (#4292)

Doc creation failed because the necessary dependencies where not
installed.

Author: DudeNr33

tox.ini

@@ -61,7 +61,6 @@ commands =
 changedir = doc/
 deps =
     -r {toxinidir}/requirements_docs.txt
-skip_install = true
 commands =
     sphinx-build -W -b html -d _build/doctrees . _build/html