Modernizing the packaging of the library

Hi,

I'm wondering if it would be alright if I went and rewrote the packaging, testing, and linting configurations using the new TOML format (`pyproject.toml`) and removed several redundant files from the repository (`requirements*.txt`, `MANIFEST.in`, etc.); Linting and other tooling changes would help with potential security and code optimization issues.

If possible, I'd also ask if I can go about replacing `setuptools` with something a little simpler: `flit`/`flit-core` does more than we need here (and is also available on Debian: https://packages.debian.org/source/forky/x32/flit).