change default python 3 env in tox to 3.5

Change-Id: I018fe4e9bae8d6b5be0b527dfc26e914ce77fb11
diff --git a/tox.ini b/tox.ini
index 5a4d525..6b0e1ae 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = py34,py27,pypy,pep8
+envlist = py3,py27,pep8
 skipsdist = True
 
 [testenv]
@@ -13,18 +13,23 @@
 commands = python setup.py test --slowest --testr-args='{posargs}'
 
 [testenv:pep8]
+basepython = python3
 commands = flake8 {posargs}
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands = python setup.py test --coverage --testr-args='{posargs}'
 
 [testenv:docs]
+basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper {posargs}
 
 [flake8]