py: Make MP_OBJ_NEW_SMALL_INT cast arg to mp_int_t itself.

Addresses issue #724.
diff --git a/py/objint.c b/py/objint.c
index e351c3f..c08bf7d 100644
--- a/py/objint.c
+++ b/py/objint.c
@@ -64,7 +64,7 @@
                 return mp_parse_num_integer(s, l, 0);
 #if MICROPY_PY_BUILTINS_FLOAT
             } else if (MP_OBJ_IS_TYPE(args[0], &mp_type_float)) {
-                return MP_OBJ_NEW_SMALL_INT((mp_int_t)(MICROPY_FLOAT_C_FUN(trunc)(mp_obj_float_get(args[0]))));
+                return MP_OBJ_NEW_SMALL_INT((MICROPY_FLOAT_C_FUN(trunc)(mp_obj_float_get(args[0]))));
 #endif
             } else {
                 // try to convert to small int (eg from bool)