Switch to hatchling build-backend

[LecrisUT]

I would recommend not using poetry, particularly since using any other PEP517 compliant build backend makes things so much easier.

I have kept the dev dependency as-is so you can simply run pip install .[dev], but I would recommend against using that because the only dependency there is pre-commit, which is better if you have it installed and managed globally instead of within the venv. Otherwise the other dependencies are split like that because test dependencies are often required by packagers, and docs can be used for the specific documentation build backend.

I would also recommend to drop Python<3.8, and even python 3.8 at this point.