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: I55118c8c80884d6f71d2730dfc26b28e5b1fdf28
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -3,6 +3,7 @@ minversion = 2.0
 envlist = py35,py27,pep8,docs
 
 [testenv]
+basepython = python3
 install_command = pip install {opts} {packages}
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
@@ -10,6 +11,9 @@ deps =
   -r{toxinidir}/requirements.txt
 commands = python setup.py testr --slowest --testr-args='{posargs}'
 
+[testenv:py27]
+basepython = python2.7
+
 [testenv:pep8]
 deps =
   -r{toxinidir}/test-requirements.txt
@@ -49,7 +53,6 @@ commands = pip-missing-reqs -d --ignore-module=oslo_policy* --ignore-module=pkg_
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt