Add Python 3.5 classifier and venv

Now that there is a passing gate job, we can claim
support for Python 3.5 in the classifier.
This patch also adds the convenience py35 venv.

Change-Id: Ifbd1147a3c8c4d6fa745d210d37a90f5ae8321dd

setup.cfg

@@ -17,6 +17,7 @@ classifier =
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.4
+    Programming Language :: Python :: 3.5
 
 [files]
 packages =

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 1.6
-envlist = py34,py27,pypy,pep8
+envlist = py35,py34,py27,pypy,pep8
 skipsdist = True
 
 [testenv]