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: Ib824a32b3135ccc6a6d64f41f5172838677b0189
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -32,12 +32,14 @@ deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 
 [testenv:pep8]
+basepython = python3
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = flake8 {posargs} hooks unit_tests tests actions lib
            charm-proof
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:func27-noop]