Run fawltydeps against itself in CI

[zz1874]

Closed #123

This PR adds a functionality of running fawltydeps against itself in the GitHub Actions CI.
While running fawltydeps, it reads in the python environment from ~/.cache/pypoetry/virtualenvs and ignores format related dependencies (black, codespell, colorama, hypothesis, mypy, nox, pylint, pytest, and types-setuptools).

Also, this PR declares setuptools as a dependency since pkg_resources has been imported to the main code.

[jherland]

Thanks for looking into this. I like the direction of these changes, and they get the job done. 👍🏻

There's one comment below, that I'd like you to reconsider before we merge this.

Another thing to think about (either in this PR or a future PR) is to not only run this FawltyDeps self-check in GitHub CI, but also make it available in our dev shell as a Nox action. This involves adding a new self_test function to noxfile.py, and changing .github/workflows/tests.yaml to instead:

run: poetry run nox --no-venv --no-install --non-interactive -s self_test -- -vv

The advantage of this (and the reason we've done this for the other CI actions) is to make it easy to run the CI actions locally. This helps make CI failures less common (since we can run the same tests locally before we push) and it also make CI failures easier to debug (because we can easily run the same command localy: nox -s self_test).

[zz1874]

Thanks for the review @jherland !
Totally agree that turning the CI check into a nox action is a smart move, as we can test it out on our own machines. So, I've gone ahead and made that change, and now the CI check runs smoothly using the new self_test function. Your input was awesome!

[jherland]

Great! Only one thing remaining, that I only realized while reading this latest version, so please don't feel bad for not catching it! 😅