devel/py-build: convert to USE_PYTHON=pep517 and use flit

[vishwin]

Summary

The commit introducing flit as the build backend is not yet in a release, but happened immediately after the 0.9.0 release. Pulling in the patch on top of the CHEESESHOP distribution makes patch(1) sad so just use the commit directly.

Test Plan

Not seeking approval to commit until framework support formally lands. Successfully packages, passes QA and works as intended.