Travis CI: Strict flake8 (#1962)

* Travis CI: Strict flake8

Turn the screws all the way down.

* updating DIRECTORY.md

* Switch from flake8 --select to --ignore

* Quotes

* IGNORE=E123,E203,E265,E266,E302,E401,E402,E712,E731,E741,E743,F811,F841,W291,W293,W503

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>