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: I28c143edde30be7c37d22c9653576df2999d71ba

tox.ini

@@ -4,5 +4,6 @@ skipsdist = True
 envlist = releasenotes
 
 [testenv:releasenotes]
+basepython = python3
 deps = -rtest-requirements.txt
 commands = bash -c tools/releasenotes_tox.sh