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[3.14] gh-137740: Clarify __del__ invocation mechanism in reference counting (GH-137741) (#138113)

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Co-authored-by: Robin Narsingh Ranabhat <robinnarsingha123@gmail.com>
Co-authored-by: Kumar Aditya <kumaraditya@python.org>
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@ -1059,7 +1059,14 @@ references to all its items, so when item 1 is replaced, it has to dispose of
the original item 1. Now let's suppose the original item 1 was an instance of a
user-defined class, and let's further suppose that the class defined a
:meth:`!__del__` method. If this class instance has a reference count of 1,
disposing of it will call its :meth:`!__del__` method.
disposing of it will call its :meth:`!__del__` method. Internally,
:c:func:`PyList_SetItem` calls :c:func:`Py_DECREF` on the replaced item,
which invokes replaced item's corresponding
:c:member:`~PyTypeObject.tp_dealloc` function. During
deallocation, :c:member:`~PyTypeObject.tp_dealloc` calls
:c:member:`~PyTypeObject.tp_finalize`, which is mapped to the
:meth:`!__del__` method for class instances (see :pep:`442`). This entire
sequence happens synchronously within the :c:func:`PyList_SetItem` call.
Since it is written in Python, the :meth:`!__del__` method can execute arbitrary
Python code. Could it perhaps do something to invalidate the reference to