Default to python3 in tox testenvs

Other than explicitly python2-based tests, we want to be using
Python 3 for new projects.

Change-Id: Idf49aab9b262cbcc6c4c203f0a2a5429894c968f

oslo.{{cookiecutter.module_name}}/tox.ini

@@ -1,7 +1,8 @@
 [tox]
-minversion = 1.8
-envlist = py35,py27,pep8
+minversion = 3.1
+envlist = py36,py27,pep8
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
 usedevelop = True
@@ -11,6 +12,7 @@ deps =
     -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
     -r{toxinidir}/test-requirements.txt
 commands = stestr run --slowest {posargs}
+basepython = python3
 
 [testenv:lower-constraints]
 basepython = python3