[3.13] Programming FAQ: Mention object.__setattr__ as a technique for delegation (GH-124617) (#124624)

Programming FAQ: Mention object.__setattr__ as a technique for delegation (GH-124617)

This is used for example by threading.local in the stdlib.
(cherry picked from commit 43979fad904bcc343f90cb526faa526c45fcbfa4)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>