requirements: fix voeventlib install to be only Python 3

requirements.txt

@@ -35,7 +35,7 @@ service_identity==17.0.0
 simplejson==3.15.0
 Sphinx==1.7.0
 twilio==6.10.3
-voeventlib==1.2
+voeventlib==1.2; python_version >= '3'
 # Do NOT upgrade pyparsing beyond 2.3.0 (even to 2.3.1) without carefully
 # testing and modifying the search query code. There were a number of
 # problematic API changes in 2.3.1 and beyond that will need to be handled