migrate to pyproject.toml and drop setuptools

[branchv]

This migrates to pyproject.toml, the modern replacement for setup.py (and setup.cfg, MANIFEST.in, etc) favoring static metadata. I used flit_core for the build backend since it's very simple and doesn't require any custom config.

Here's how I checked the difference between the two builds:

$ uv build -o /tmp/before
$ git switch pyproject && uv build -o /tmp/after
$ uvx diffoscope --exclude-directory-metadata=recursive /tmp/before/*.tar.gz /tmp/after/*.tar.gz

Also, drop setuptools from dependencies since it's unused after #384