objgenerator: Can optimize StopIteration to STOP_ITERATION only if arg is None.

Unfortunately, MP_OBJ_STOP_ITERATION doesn't have means to pass an associated
value, so we can't optimize StopIteration exception with (non-None) argument
to MP_OBJ_STOP_ITERATION.

py/objgenerator.c

@@ -171,11 +171,13 @@ STATIC mp_obj_t gen_resume_and_raise(mp_obj_t self_in, mp_obj_t send_value, mp_o
             // of mp_iternext() protocol, but this function is called by other methods
             // too, which may not handled MP_OBJ_STOP_ITERATION.
             if (mp_obj_is_subclass_fast(mp_obj_get_type(ret), &mp_type_StopIteration)) {
+                mp_obj_t val = mp_obj_exception_get_value(ret);
+                if (val == mp_const_none) {
                     return MP_OBJ_STOP_ITERATION;
-            } else {
-                nlr_raise(ret);
                 }
             }
+            nlr_raise(ret);
+    }
 }
 
 STATIC mp_obj_t gen_instance_iternext(mp_obj_t self_in) {