Update to a PEP621 project configuration

[purepani]

PEP621 defines a format for pyproject.toml to be shared across package managers, all package managers aside from poetry currently support it. Since this is the accepted way to package a python package now, updating to it would allow for support for more modern tools. I have a local clone of this repo where I used pdm to convert the setup.py file, and as far as I'm able to test, it works pretty well. If pdm is the package manager you would like to use, I can clean up the local repo and submit a PR.