Replace PyObject_Del with PyObject_Free

[vstinner]

PyObject_Del() is just a alias to PyObject_Free() kept for backward compatibility. Use directly PyObject_Free() instead.

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📚 Documentation preview 📚: https://cpython-previews--122453.org.readthedocs.build/

[lazka]

This is an improvement, but now the docs are inconsistent/contradicting:

• tp_dealloc docs say that PyObject_Free() should be used after PyObject_New() for example. PyObject_Free() docs states that this is undefined behavior.
• PyObject_Del() docs say that is should be called from tp_dealloc, while tp_dealloc docs say PyObject_Free() should be called.

[vstinner]

I wrote a follow-up PR to update the doc: #122597