devel/py-pathtools: Fix build on Python 3.12+

• The patch provided [1] comes from

https://github.com/gorakhargosh/pathtools/issues/13

and it is already used by other projects: e.g.

https://salsa.debian.org/python-team/packages/python-pathtools/-/blob/debian/master/debian/patches/python-3.12-support.patch?ref_type=heads

• Convert port to PEP517
• Pet portclippy

PR: 294676 [1]