`build(build-system)`: Remove `setuptools` build dependency

[tony]

via #493 by @tjni:

This was added in 3bc03fb to support pip install -e . but I noticed that (per python-poetry/poetry#34 (comment)), this is fixed in pip >= 21.3.

For context, I am working on this package in nixpkgs and investigating if setuptools is really needed as a build dependency.