requirements: install VOEventLib from PyPI

For Python 2, we install VOEventLib from a git fork since upstream
is not Python 2-compatible.  But we don't need to do that for Python 3!

requirements.txt

@@ -35,6 +35,7 @@ service_identity==17.0.0
 simplejson==3.15.0
 Sphinx==1.7.0
 twilio==6.10.3
+voeventlib==1.2
 # 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