Use python3 for tox linting
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
Also, add tox-linters job to gate so that changes like this can merge.
The files section limit what runs - and if *nothing* run, there's no +2
from CI. Another reason to add this job is that we run voting jobs in
both check and gate, and this one was only in check.