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.

Review that added the gate jobs:
https://review.openstack.org/#/c/336272/

Change-Id: I97ef7eef2d6adaec6bd1cd978b7e357c8560eba0

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

@@ -1,5 +1,5 @@
 [tox]
-envlist = pep8,py34,py27,docs
+envlist = pep8,py35,py34,py27,docs
 
 [testenv]
 install_command = pip install -U {opts} {packages}