set default python to python3

Set the default python to python3 except for the py27 environment. We
have to set that explicitly to override the new default.

Change-Id: I45878900100ca51b017fec0ba2e11c78fc743f4a

tox.ini

@@ -4,6 +4,7 @@ envlist = py34,py27,pypy,pep8
 skipsdist = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command = pip install {opts} {packages}
 setenv =
@@ -14,6 +15,9 @@ deps =
   -r{toxinidir}/test-requirements.txt
 commands = python setup.py test --slowest --testr-args='{posargs}'
 
+[testenv:py27]
+basepython = python2.7
+
 [testenv:pep8]
 commands = flake8 {posargs}