Fix test (#919)

> AttributeError: 'called_once_with' is not a valid assertion.
> Use a spec for the mock if 'called_once_with' is meant to be an
attribute..
> Did you mean: 'assert_called_once_with'?

Ref python/cpython#100690

Author: mthrok

tests/spdl_unittest/io/transfer_test.py

@@ -127,7 +127,7 @@ def _test():
         # check 1
         mock_Stream.assert_called_once_with(device)
         # check 2
-        mock_stream_func.called_once_with(mock_stream_obj)
+        mock_stream_func.assert_called_once_with(mock_stream_obj)
         # Check 5
         mock_stream_obj.synchronize.assert_called_once()