Use python3 by default

Use the shiny new tox functionality to ignore basepython conflicts so
we get sane behavior.

Change-Id: I304afabc670a2a61297b11ab3a5934e56cc4e1b8

tox.ini

@@ -1,7 +1,8 @@
 [tox]
-minversion = 1.8
+minversion = 3.1
 envlist = py34-constraints,py27-constraints,pypy-constraints,pep8-constraints
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
 usedevelop = True
@@ -12,6 +13,7 @@ setenv =
    VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/test-requirements.txt
 commands = stestr run --slowest {posargs}
+basepython = python3
 
 [testenv:common-constraints]
 install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}