py: Use mp_not_implemented consistently for not implemented features.
diff --git a/py/objarray.c b/py/objarray.c
index b14ad5a..f5ea794 100644
--- a/py/objarray.c
+++ b/py/objarray.c
@@ -365,8 +365,7 @@
         } else if (MP_OBJ_IS_TYPE(index_in, &mp_type_slice)) {
             mp_bound_slice_t slice;
             if (!mp_seq_get_fast_slice_indexes(o->len, index_in, &slice)) {
-                nlr_raise(mp_obj_new_exception_msg(&mp_type_NotImplementedError,
-                    "only slices with step=1 (aka None) are supported"));
+                mp_not_implemented("only slices with step=1 (aka None) are supported");
             }
             if (value != MP_OBJ_SENTINEL) {
                 #if MICROPY_PY_ARRAY_SLICE_ASSIGN