devel/py-flit-core: convert to self-hosting bootstrap

This is the first port used to bootstrap the rest of the PEP-517
tooling. 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/D34786