> ITSM it's not the TextIOWrapper but the detect_encoding fails and throws an error.

Oh, right. But TextIOWrapper can fail for differen reasons. For example, CTRL+c may send KeyboardInterrupt.

Try for example:

    with unittest.mock.patch.object(tokenize, '_builtin_open') as mock_open:
        mock_file = mock_open.return_value
        mock_file.tell.side_effect = OSError
        mock_file.readline.return_value = b''

        tokenize.open(fn)

This example raises an OSError in TextIOWrapper on file.tell(), and file.close() is not called.