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: Ibaf75b2da94c8ba4e0c1f4b3e60992c1a3f41001
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -3,6 +3,7 @@ minversion = 2.0
 envlist = py35,py27,pep8
 
 [testenv]
+basepython = python3
 setenv =
     VIRTUAL_ENV={envdir}
     BRANCH_NAME=master
@@ -14,6 +15,9 @@ commands =
   python setup.py test --coverage --coverage-package-name=oslo_log --slowest --testr-args='{posargs}'
   coverage report --show-missing
 
+[testenv:py27]
+basepython = python2.7
+
 [testenv:pep8]
 commands =
   flake8
@@ -57,7 +61,6 @@ commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasen
 
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt