I finally spent some time figuring out a minimal way of developing and packaging a Python library using a pyproject.toml file
It turns out you can do it with just a pyproject.toml file - no setup.py, no setup.cfg - and the only tools you need are pip and build, with no need to use Poetry/Hatch/Flit/etc for the simplest case
In particular I'm not at all confident I understand how pip and build automatically discover Python modules and packages - a note on my confusion about that here: https://til.simonwillison.net/python/pyproject#user-content-it-finds-the-python-files-for-you