[3.11] gh-107432 Update Porting Python 2 Code to Python 3 how-to (GH-107434) (#108410)

https://docs.python.org/3/howto/pyporting.html was written for another time. In this patch:

- material that frames Python 3 as "new" is removed
- descriptions and directions have been trimmed
(cherry picked from commit 809ea7c4b6c2b818ae510f1f58e82b6b05ed4ef9)

Co-authored-by: Daniele Procida <daniele@vurt.org>