py: Make MP_OBJ_NEW_SMALL_INT cast arg to mp_int_t itself.

Addresses issue #724.
diff --git a/py/objset.c b/py/objset.c
index 48d037f..f17628d 100644
--- a/py/objset.c
+++ b/py/objset.c
@@ -476,7 +476,7 @@
     mp_obj_set_t *self = self_in;
     switch (op) {
         case MP_UNARY_OP_BOOL: return MP_BOOL(self->set.used != 0);
-        case MP_UNARY_OP_LEN: return MP_OBJ_NEW_SMALL_INT((mp_int_t)self->set.used);
+        case MP_UNARY_OP_LEN: return MP_OBJ_NEW_SMALL_INT(self->set.used);
         default: return MP_OBJ_NULL; // op not supported
     }
 }