Refactor `mappingproxy_new_impl` method to use `PyDictProxy_New`

[sobolevn]

They are located next to each other, use identical code.
They should be refactored to be the same thing:

cpython/Objects/descrobject.c

Lines 1265 to 1296 in 2670cb0

	static PyObject *
	mappingproxy_new_impl(PyTypeObject *type, PyObject *mapping)
	/*[clinic end generated code: output=65f27f02d5b68fa7 input=c156df096ef7590c]*/
	{
	mappingproxyobject *mappingproxy;
	if (mappingproxy_check_mapping(mapping) == -1)
	return NULL;
	mappingproxy = PyObject_GC_New(mappingproxyobject, &PyDictProxy_Type);
	if (mappingproxy == NULL)
	return NULL;
	mappingproxy->mapping = Py_NewRef(mapping);
	_PyObject_GC_TRACK(mappingproxy);
	return (PyObject *)mappingproxy;
	}
	PyObject *
	PyDictProxy_New(PyObject *mapping)
	{
	mappingproxyobject *pp;
	if (mappingproxy_check_mapping(mapping) == -1)
	return NULL;
	pp = PyObject_GC_New(mappingproxyobject, &PyDictProxy_Type);
	if (pp != NULL) {
	pp->mapping = Py_NewRef(mapping);
	_PyObject_GC_TRACK(pp);
	}
	return (PyObject *)pp;
	}

Linked PRs

• gh-152546: Refactor mappingproxy.__new__ to use PyDictProxy_New #152547