lang/python313: add

What's new: https://docs.python.org/3/whatsnew/3.13.html

Most notably, introduce free-threaded mode. As this mode changes
the ABI, the entire distribution lives in a separate hierarchy, and
thus a new child port (lang/python313t) and FLAVOR (py313t) are
introduced.

Co-authored-by: vishwin
PR: 274671
Differential Revision: https://reviews.freebsd.org/D49680