tox is apparently incompatible with poudriere style testing, so just
run what tox would have done directly.

In this case, the flake8 test always fails due to a version conflict:

pkg_resources.ContextualVersionConflict: (pyflakes 2.0.0 (/usr/local/lib/python3.6/site-packages), Requirement.parse('pyflakes<1.7.0,>=1.5.0'), {'flake8'})