Not using `__class_getitem__()` fallback if there is a non-subcriptable metaclass was caused by a certain asymmetry between how `PySequenceMethods` and `PyMappingMethods` are used in `PyObject_GetItem`. This PR removes this asymmetry. No tests failed, so I assume it was not intentional.
(cherry picked from commit ac28147e78)
Co-authored-by: Ivan Levkivskyi <levkivskyi@gmail.com>