fixup! buildout: Fix Sphinx install on Python2

It is not only py2 as build on py3 is also affected, e.g.

https://travis-ci.com/github/zopefoundation/ZODB/jobs/499826281

And even though Sphinx is going to be ready for newer docutils,
sphinx_rtd_theme requires docutils<0.17 for now:

https://github.com/readthedocs/sphinx_rtd_theme/blob/2026bb61ddf85a672141127b162d6f9181603725/setup.py#L119-L122

buildout.cfg

@@ -9,12 +9,13 @@ parts =
 [versions]
 # Avoid breakage in 4.4.5:
 zope.testrunner = >= 4.4.6
+# sphinx_rtd_theme depends on docutils<0.17
+docutils = < 0.17
 
 [versions:python2]
 Sphinx = < 2
 pygments = < 2.6
 sphinxcontrib-websupport = < 1.2
-docutils = < 0.17
 
 [test]
 recipe = zc.recipe.testrunner