[3.12] gh-104212: Explain how to port imp.load_source() (GH-105978) (#106083)

gh-104212: Explain how to port imp.load_source() (GH-105978)

Explain how to port removed imp.load_source() to importlib in What's
New in Python 3.12.
(cherry picked from commit 18a7c86697493510993e43bafe8bd4046928bec5)

Co-authored-by: Victor Stinner <vstinner@python.org>