The `Include/object.h` header file became quite big (1260 lines of C code). A large part of it is just reference count management. I propose adding a new `Include/refcount.h` header file to ease the navigation in `Include/object.h` and ease the maintenance of the C API.

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### Linked PRs
* gh-119854
* gh-119860
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[C API] Split large object.h file: create refcount.h
This is a GitHub Issue
repo:cpython
owner:python
Title : [C API] Split large object.h file: create refcount.h
Issue date:

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The `Include/object.h` header file became quite big (1260 lines of C code). A large part of it is just reference count management. I propose adding a new `Include/refcount.h` header file to ease the navigation in `Include/object.h` and ease the maintenance of the C API.

<!-- gh-linked-prs -->
### Linked PRs
* gh-119854
* gh-119860
<!-- /gh-linked-prs -->

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