fix tox python3 overrides

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

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.

Add a new option from tox 3.1 to ignore this basepython override
conflict, and update the minver accordingly.

Change-Id: I89479c30c4118e46aed51f0f52630b704546892c
Task: #23643

tox.ini

@@ -1,10 +1,12 @@
 [tox]
-minversion = 1.6
-envlist = py35,py27,pep8
+minversion = 3.1
+envlist = py35,py36,py37,py27,pep8
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
 usedevelop = True
+basepython = python3
 install_command = pip install -U {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}