use python3 in tox

Add a py27 environment to override the basepython value set in testenv
to ensure those tests still actually run under python 2.

Change-Id: I55c97908e95a7e1d5c15a16caf81fca13da215d9
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -4,6 +4,7 @@ envlist = py27,pep8
 skipsdist = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command = pip install {opts} {packages}
 setenv =
@@ -14,6 +15,9 @@ deps =
   -r{toxinidir}/test-requirements.txt
 commands = python setup.py test --slowest --testr-args='{posargs}'
 
+[testenv:py27]
+basepython = python2.7
+
 [testenv:pep8]
 commands = flake8 {posargs}