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: Ia540b70e648d585aa0de017f72ceb96214b67e27
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}
@@ -11,6 +12,9 @@ deps =
 distribute = False
 commands = python setup.py testr --testr-args='{posargs}'
 
+[testenv:py27]
+basepython = python2.7
+
 [testenv:venv]
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
@@ -45,7 +49,6 @@ deps =
 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