Avoid refcounting on code objects during frame creation/destruction

The __code__ attribute of function objects is assignable.
As a consequence, frames must hold a strong reference to the code object in case the reference held by the function is lost.

Ideally, the __code__ attribute would be read-only, but to fix that would be a multi-year deprecation effort.

Until that happens, if ever, we can replace the strong reference with a borrowed reference to the code object as long as code object is guaranteed to outlive the frame.
Since the function will outlive the frame, if the function object keeps references to its prior __code__ attributes
when __code__ is assigned, then the code object will always outlive the frame.

This will need an extra field in each function, but the cost of one assignment to NULL during function creation, is much less than the cost of an incref and a decref on every call to that function.

The only downside is that in some obscure cases the lifetime of a few code objects will be extended.
Since the free-threading build makes all code objects immortal, this could actually be a big saving for the free-threading build.

Linked PRs

• gh-152955

Beginne mit der Durchsicht des verknüpften PR gh-152955 und verfolge anschließend Funktionsobjekte, Frames und Codeobjekte rund um das Attribut code sowie die Erstellung und Zerstörung von Frames. Als abgeschlossen gilt die Aufgabe, wenn nachgewiesen ist, dass Codeobjekte ihre Frames überleben, ohne Referenzzähloperationen pro Aufruf zu benötigen, und die relevanten CPython-Tests weiterhin bestehen.