Switch from poetry to plain setuptools and build

[timofurrer]

I don't feel there is a need for poetry for this project, especially because it's a library supporting many Python versions.

Lets switch modern setuptools / build setup.

[sisp]

IMO, Poetry's advantages also for library projects are a unified CLI, dev dependencies, and a lock file (which is a matter of taste for libraries, but it helps in making a dev environment reproducible).

[timofurrer]

I don't have strong feelings about it actually, we could leave as-is. However, last time I used poetry it was a pain to test for multiple Python versions and maintain multiple venvs with it.

[timofurrer]

Oh and yes, we'd still need a lock file :)