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: I64799bb18c6c5848d23378e163481ad0fe1eea2b
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -4,6 +4,7 @@ envlist = py35,py27,pypy,pep8
 skipsdist = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command = pip install {opts} {packages}
 setenv =
@@ -15,6 +16,9 @@ deps =
        -r{toxinidir}/test-requirements.txt
 commands = python setup.py testr --slowest --testr-args='{posargs}'
 
+[testenv:py27]
+basepython = python2.7
+
 [testenv:pep8]
 commands =
   flake8
@@ -83,7 +87,6 @@ enable-extensions = H106,H203
 import_exceptions = castellan.i18n
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt