fix tox python3 overrides

We want to default to running all tox environments under python 3, so
set the basepython value in each environment.

We do not want to specify a minor version number, because we do not
want to have to update the file every time we upgrade python.

We do not want to set the override once in testenv, because that
breaks the more specific versions used in default environments like
py35 and py36.

Change-Id: I45a7155785ce37dbeb8c77d409437d65bcdc479b

scripts/marathon-health-check-testing/HealthCheckBencher.py

@@ -13,6 +13,7 @@ from marathon.models import MarathonHealthCheck
 from Queue import Empty
 from Queue import Queue
 from random import random
+import six
 from threading import Thread
 from time import sleep
 from urllib2 import urlopen
@@ -198,7 +199,7 @@ class HealthCheckBencher(object):
     def start_test(self):
         task_list = self.get_tasks()
         for action in self.action_list:
-            if isinstance(action, basestring):
+            if isinstance(action, six.text_type):
                 if action.startswith('sleep='):
                     amount = int(action.split('=')[1])
                     sleep(60*amount)

tox.ini

@@ -5,7 +5,6 @@ skipsdist = True
 
 [testenv]
 usedevelop = True
-basepython = python2.7
 # --ignore-installed is added to workaround problem with pip 8.0 and argparse
 # https://github.com/pypa/pip/issues/3404 and
 # https://github.com/pypa/pip/issues/3384
@@ -21,12 +20,15 @@ commands =
   python setup.py test --slowest --testr-args='{posargs}'
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:pep8]
+basepython = python3
 commands = flake8
 
 [testenv:docs]
+basepython = python3
 commands =
     python setup.py build_sphinx