devel/py-installer: convert to USE_PYTHON=pep517

This is the second port used to bootstrap PEP-517 tooling and the
default staging driver for ports built under USE_PYTHON=pep517.
python@ will be maintaining this going forward as infrastructure.

https://wiki.freebsd.org/Python/PEP-517

With hat: python
Approved by: maintainer timeout, fluffy (mentor)
Differential Revision: https://reviews.freebsd.org/D34789