python: add tox support

python: add tox support

This is intended to be a manually run, non-CI script.

Use tox to test the linters against all python versions from 3.6 to
3.10. This will only work if you actually have those versions installed
locally, but Fedora makes this easy:

> sudo dnf install python3.6 python3.7 python3.8 python3.9 python3.10

Unlike the pipenv tests (make venv-check), this pulls "whichever"
versions of the python packages, so they are unpinned and may break as
time goes on. In the case that breakages are found, setup.cfg should be
amended accordingly to avoid the bad dependant versions, or the code
should be amended to work around the issue.

With confidence that the tests pass on 3.6 through 3.10 inclusive, add
the appropriate classifiers to setup.cfg to indicate which versions we
claim to support.

Tox 3.18.0 or above is required to use the 'allowlist_externals' option.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Tested-by: Cleber Rosa <crosa@redhat.com>
Message-id: 20210527211715.394144-31-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>