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: Ib129db4e523fc5983a246c74f7f057bade0b7093
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -8,14 +8,16 @@ deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:pep8]
-basepython = python2.7
+basepython = python3
 commands =
     doc8 README.rst doc/source specs
 
 [testenv:docs]
+basepython = python3
 # TODO(milan): remove the version pin once feedformatter supports python3
 basepython = python2.7
 commands = python setup.py build_sphinx