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: Iaec0c9e88cb38ab876b5bc957f53989615cab246
diff --git a/tox.ini b/tox.ini
index cff222d..a33101b 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-envlist = pep8,py34,py27
+envlist = pep8,py35,py34,py27
 minversion = 2.3.1
 skipsdist = True