devel/py-setuptools: convert to USE_PYTHON=pep517

The previous method of building/bootstrapping setuptools via the
built-in distutils module no longer works in Python 3.12+ as it has
been removed [0]. Since USE_PYTHON=pep517 has its own bootstrapping
process, use this method to build setuptools.

This allows newer Python distributions/interpreters to land in the
tree, for the purpose of having buildable packages.

While here, remove a slew of dead code and adjust WWW

[0] https://peps.python.org/pep-0632/

PR: 271673, 274671