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: I8557195409a559950da6c95aece5787195f31ad6
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -3,6 +3,7 @@ minversion = 2.0
 envlist = py35,py27,pep8,pip-missing-reqs
 
 [testenv]
+basepython = python3
 install_command = pip install {opts} {packages}
 whitelist_externals = bash
                       env
@@ -29,6 +30,7 @@ commands = pip install SQLAlchemy>=0.9.0,!=0.9.5,<1.0.0
             python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:py27]
+basepython = python2.7
 commands =
     env TEST_EVENTLET=0 bash tools/pretty_tox.sh '{posargs}'
     env TEST_EVENTLET=1 bash tools/pretty_tox.sh '{posargs}'
@@ -74,7 +76,6 @@ deps = pip_missing_reqs
 commands = pip-missing-reqs -d --ignore-module=oslo_db* --ignore-module=pkg_resources --ignore-file=oslo_db/tests/* oslo_db
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt