[3.12] gh-112358: Fix Python 3.12 regression with subclassing struct.Struct (GH-11242...

[3.12] gh-112358: Fix Python 3.12 regression with subclassing struct.Struct (GH-112424) (#112426)

* [3.12] gh-112358: Fix Python 3.12 regression with subclassing struct.Struct. (GH-112424)

Revert commit c8c0afc7137ab9f22bf59d591084948ca967c97c (PR GH-94532),
which moved `struct.Struct` initialisation from `Struct.__init__` to `Struct.__new__`.
This caused issues with code in the wild that subclasses `struct.Struct`..
(cherry picked from commit 9fe60340d7e8dc22b3aec205c557bc69a1b2d18c)

Co-authored-by: Mark Dickinson <dickinsm@gmail.com>
* Remove unrelated test