And I should've been more upfront about the real reason for suggesting setuptools: there seem to be a number of build tools that support pyproject.toml, including flit, poetry and setuptools (and I'm sure I've seen at least one other). For me, at least, when I was making a small library recently, it was an overwhelming choice for a part of my project that feels like just admin rather than core business logic. I came close to giving up and just using setup.py with `setup()`. At least setuptools with pyproject.toml is a choice that feels safe; it may not be the best, but it will certainly be good enough that I'm unlikely to regret it later, so I didn't need to spend a lot of time looking at the detailed pros and cons of all the choices.