[3.9] gh-92417: `stdtypes` docs: delete discussion of Python 2 differences (GH-92423...

[3.9] gh-92417: `stdtypes` docs: delete discussion of Python 2 differences (GH-92423) (GH-92474)

Given that 2.7 has now been end-of-life for two and a half years,
I don't think we need such a detailed explanation here anymore of
the differences between Python 2 and Python 3.
(cherry picked from commit 8efda1e7c6343b1671d93837bf2c146e4cf77bbf)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Automerge-Triggered-By: GH:serhiy-storchaka