gh-107432 Update Porting Python 2 Code to Python 3 how-to (GH-107434)

https://docs.python.org/3/howto/pyporting.html#porting-python-2-code-to-python-3 was written for another time. In this patch:

- material that frames Python 3 as "new" is removed
- descriptions and directions have been trimmed
1 file changed