py/objsingleton: Use mp_generic_unary_op for singleton objects.

So these types more closely match NoneType, eg they can be hashed, like in
CPython.

py/objsingleton.c

@@ -47,6 +47,7 @@ const mp_obj_type_t mp_type_singleton = {
     { &mp_type_type },
     .name = MP_QSTR_,
     .print = singleton_print,
+    .unary_op = mp_generic_unary_op,
 };
 
 const mp_obj_singleton_t mp_const_ellipsis_obj = {{&mp_type_singleton}, MP_QSTR_Ellipsis};