python.mk: introduce USE_PYTHON=pep517 for PEP-517 support

USE_PYTHON=pep517 takes no arguments. Operation is similar to
USE_PYTHON=distutils, although the build backend specified in
pyproject.toml is to be specified in BUILD_DEPENDS explicitly. A
usage guide and implementation primer is available at:
https://wiki.freebsd.org/Python/PEP-517

With hat: python
Approved by: fluffy (mentor)
Co-authored by: yuri
PR: 255722
Differential Revision: https://reviews.freebsd.org/D36290