> what file(s) is/are the sys.path code located in?

If I understand it correctly, sys.path is handled in importlib._bootstrap_external.PathFinder.find_spec(). I can patch PathFinder for handling path-like objects: https://github.com/yan12125/cpython/commit/e3fd473b54cbb368533e651fd896bbc813a43924

Here's an example usage:

# t.py
import pathlib
import sys

sys.path.append(pathlib.Path('foo'))

import s

# foo/s.py
print(123)