refactor: switch to Poetry for env+dep management

[crccheck]

I've been using Poetry because it takes the hassle out of virtualenv management and has sane defaults that just work for building artifacts and uploading to PyPI.

Because I had to redo how tests were run, I went ahead and removed the Tox testing requirement too and so I had to redo the Github Actions for testing too.

One thing I didn't anticipate is that Github Actions caching doesn't work for Poetry, only with requirements.txt and Pipenv https://github.blog/changelog/2021-11-23-github-actions-setup-python-now-supports-dependency-caching/

Verifying the change

I compared python setup.py build vs poetry build and the only difference was some top level meta differences and Poetry added the tests directory which is fine. Both have the .html templates which is the important thing.