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: Ia7b1fb88ac108e1e8fd6ed963967e731ae3ca5a4
Closes-Bug: #1603006

releasenotes/notes/add-py35-f0ddb519029f5ee3.yaml

+---
+prelude: >
+  Infra team will start running python35 jobs
+  target to replace current python34 jobs
+  This is due to the change from Ubuntu Trusty to
+  Xenial, where python3.5 will be available.
+features:
+  - Add Python 3.5 classifier and venv

setup.cfg

@@ -15,6 +15,8 @@ classifier =
     Programming Language :: Python
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
+    Programming Language :: Python :: 3.4
+    Programming Language :: Python :: 3.5
 
 [files]
 packages =

tox.ini

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