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: Ie23c9361218a3ecdedd7682fe4dc6236994ac31d
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -3,6 +3,7 @@ minversion = 2.0
 envlist = py35,py27,pep8
 
 [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}
@@ -15,6 +16,9 @@ commands =
   python setup.py testr --slowest --testr-args='(?!tests.test_functional_eventlet)tests {posargs}'
   env TEST_EVENTLET=1 python setup.py testr --slowest --testr-args='tests.test_functional_eventlet'
 
+[testenv:py27]
+basepython = python2.7
+
 [testenv:pep8]
 deps =
   -r{toxinidir}/test-requirements.txt
@@ -42,10 +46,10 @@ show-source = True
 exclude = .tox,dist,doc,*.egg,build
 
 [testenv:benchmark]
+basepython = python2.7
 commands = python benchmark/benchmark.py
 
 [testenv:benchmark3]
-basepython = python3
 commands = python3 benchmark/benchmark.py
 
 [testenv:pip-missing-reqs]
@@ -61,7 +65,6 @@ deps = -r{toxinidir}/doc/requirements.txt
 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