py: Use mp_not_implemented consistently for not implemented features.
diff --git a/py/objstrunicode.c b/py/objstrunicode.c
index 0bb6e04..1744925 100644
--- a/py/objstrunicode.c
+++ b/py/objstrunicode.c
@@ -178,8 +178,7 @@
             mp_obj_t ostart, ostop, ostep;
             mp_obj_slice_get(index, &ostart, &ostop, &ostep);
             if (ostep != mp_const_none && ostep != MP_OBJ_NEW_SMALL_INT(1)) {
-                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");
             }
 
             const byte *pstart, *pstop;