Set tox basepython to python2

bashate isn't working under python3 so we pin to python2.

Change-Id: Idbff334bec75bdbb7f7e46a164d8904768ef5881

tox.ini

@@ -4,6 +4,7 @@ minversion = 1.6
 skipsdist = True
 
 [testenv]
+basepython = python2
 install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/test-requirements.txt