tox.ini: add support for Python 3.5

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 downloadcache = {toxworkdir}/cache/
-envlist = py26, py27, pypy, py27-noext, pypy-noext, py34, py34-noext
+envlist = py26, py27, pypy, py27-noext, pypy-noext, py34, py34-noext, py35, py35-noext
 
 [testenv]
 deps =
@@ -18,3 +18,6 @@ commands = make check-noextensions
 
 [testenv:py34-noext]
 commands = make check-noextensions
+
+[testenv:py35-noext]
+commands = make check-noextensions