gh-89653: PEP 670: Functions don't cast pointers (#91697)

In the limited C API version 3.11 and newer, the following functions
no longer cast their object pointer argument with _PyObject_CAST() or
_PyObject_CAST_CONST():

* Py_REFCNT(), Py_TYPE(), Py_SIZE()
* Py_SET_REFCNT(), Py_SET_TYPE(), Py_SET_SIZE()
* Py_IS_TYPE()
* Py_INCREF(), Py_DECREF()
* Py_XINCREF(), Py_XDECREF()
* Py_NewRef(), Py_XNewRef()
* PyObject_TypeCheck()
* PyType_Check()
* PyType_CheckExact()

Split Py_DECREF() implementation in 3 versions to make the code more
readable.

Update the xxlimited.c extension, which uses the limited C API
version 3.11, to pass PyObject* to these functions.