use python3 in tox

Set basepython once and override it for the py27 environment.

Change-Id: I7dfbe8d213f2780cb2808de9b9bebb079da50901
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -4,6 +4,7 @@ envlist = pep8,bash8
 skipsdist = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 # Every entry needs new line ...
 whitelist_externals = bash
@@ -22,6 +23,9 @@ deps = -r{toxinidir}/requirements.txt
 commands =
     find . -type f -name "*.pyc" -delete
 
+[testenv:py27]
+basepython = python2.7
+
 [testenv:venv]
 commands = {posargs}