Set base environment as python3

We are python3 all the way down now.

Change-Id: Ibdb2bffc3f47ba35aa970cc0520075186c848299

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 1.6
 skipsdist = True
-envlist = pep8, py27
+envlist = pep8,py35
 
 [testenv]
 # Set STATSD env variables so that statsd code paths are tested.
@@ -17,6 +17,7 @@ commands =
   python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:pep8]
+basepython = python3
 commands = flake8 nodepool
 
 [testenv:bindep]
@@ -28,6 +29,7 @@ deps = bindep
 commands = bindep test
 
 [testenv:cover]
+basepython = python3
 commands =
   python setup.py testr --coverage
 
@@ -36,6 +38,7 @@ commands =
   python setup.py build_sphinx
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [flake8]