gh-89828: Do not relay the __class__ attribute in GenericAlias (#93754)

list[int].__class__ returned type, and isinstance(list[int], type)
returned True. It caused numerous problems in code that checks
isinstance(x, type).

Objects/genericaliasobject.c

@@ -583,6 +583,7 @@ ga_vectorcall(PyObject *self, PyObject *const *args,
 }
 
 static const char* const attr_exceptions[] = {
+    "__class__",
     "__origin__",
     "__args__",
     "__unpacked__",