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

[tony]

via https://github.com/tmux-python/libtmux/pull/493 by @tjni:

This was added in https://github.com/tmux-python/libtmux/commit/3bc03fbcaa0c9d7719d9c537645d299b016f8d36 to support pip install -e . but I noticed that (per https://github.com/python-poetry/poetry/issues/34#issuecomment-1193722065), 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.