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: Iedf29819e1f56e7524bb213c38b8d88d0c5af8e3
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -4,6 +4,7 @@ skipsdist = True
 envlist = bashate
 
 [testenv]
+basepython = python3
 usedevelop = False
 install_command = pip install {opts} {packages}