Switch from setuptools to flit

[ronaldoussoren]

altgraph currently uses setuptools, use flit instead.

Doing this is easy enough, but requires dropping python 2 support and I don't know when I'll do that.

[flying-sheep]

Python 2 is already unsupported, so now is a good time.

[jgarte]

Hi, why would it be good to use flit instead of setuptools? 🦆

[flying-sheep]

Simplicity. Setuptools is old, and therefore has a lot of competing ways to do things, outdated tutorials, slowness, verbose logging and so on.

Build backends like Flit, Hatch, PDM, … are much less bespoke, built for pyproject.toml exclusively, and therefore faster and easier to learn.

One could also use pyproject.toml to configure setuptools and have some of the advantages, but if you run into problems, “how to do X with setuptools” will yield less helpful responses because of all the history.