Refactor atexit.pyx

[cxzhong]

I have refactor the atexit.pyx since for python 3.14, the changes of atexit is so much for supporting the nogil version.

Python<=3.13 Implementation:

• The atexit module stores callbacks in a C array of structs (atexit_callback).
• The structure is defined in C as:

typedef struct {  PyObject *func;    PyObject *args;    PyObject *kwargs;} atexit_callback;

• Callbacks are stored in the interp->atexit.callbacks field as a C array
• The array can be directly accessed from Cython code using pointer arithmetic

Python 3.14 Implementation:

• The atexit module was refactored to use a Python PyList object instead of a C array.
• The structure is now:

state.callbacks = [(func, args, kwargs), ...]  # A Python list of tuples

• In the C implementation, callbacks are managed with:

PyObject *callbacks;  // This is now a PyList

• Callbacks are inserted at the beginning (LIFO order) using PyList_Insert(state->callbacks, 0, callback)

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

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[tobiasdiez]

I would prefer to simply do nothing (or throw a 'is deprecated/removed' exception) in atexit for Python >= 3.13 (since it's not needed in this case as explained in #40890).

[cxzhong]

I would prefer to simply do nothing (or throw a 'is deprecated/removed' exception) in atexit for Python >= 3.13 (since it's not needed in this case as explained in #40890).

I just tested the doctest in atexit.pyx is OK but the order changed. The output is reversed.

[cxzhong]

I would prefer to simply do nothing (or throw a 'is deprecated/removed' exception) in atexit for Python >= 3.13 (since it's not needed in this case as explained in #40890).

I also think it is useless in Python 3.13+. We can edit the build system to exclude this. If Python version bigger than 3.13. But we need to modify forker.py, maybe we can do this when we drop support of python 3.12

[Copilot]

Pull Request Overview

This PR refactors the atexit.pyx module to support both Python ≤3.13 and Python 3.14+, which changed how atexit callbacks are stored internally. The refactoring provides a uniform interface for accessing atexit callbacks across different Python versions.

• Adds version-specific implementations for accessing atexit callbacks
• Introduces C functions that handle both the legacy C array format (Python ≤3.13) and the new PyList format (Python 3.14+)
• Updates the _get_exithandlers() function to work with both storage formats while maintaining consistent behavior

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[Copilot]

Returning NULL from a dummy function could cause segmentation faults if accidentally called. Consider adding an assertion or error handling to make accidental calls more obvious during development.