Remove setuptools from pyproject.toml

[tjni]

python-poetry/poetry#3153 has been closed, so is this still necessary? My motivation is to avoid including setuptools as a dependency for this package in nixpkgs.

• If you are a maintainer (only @nyanpasu64 at the moment), make sure to edit the changelog if needed. Otherwise, a maintainer will edit the changelog.

[nyanpasu64]

might as well, it passes CI. hope it doesn't break anything.