buildout: pin sphinxcontrib-websupport to < 1.2 on python2

Starting from 1.2.0 sphinxcontrib-websupport officially only supports python3.
Since 1.2.4 it depends on sphinxcontrib-serializinghtml which can not even be
imported on python2

buildout.cfg

@@ -56,6 +56,7 @@ eggs = Sphinx
 eggs =
     Sphinx < 2
     pygments < 2.6
+    sphinxcontrib-websupport < 1.2
 
 [sphinx]
 recipe = zc.recipe.egg