gh-111644: Fix asyncio test_unhandled_exceptions()

Fix test_unhandled_exceptions() of test_asyncio.test_streams: break
explicitly a reference cycle.

Author: vstinner

Lib/test/test_asyncio/test_streams.py

@@ -1124,6 +1124,8 @@ async def handle_echo(reader, writer):
 
         self.assertEqual(messages[0]['message'],
                          'Unhandled exception in client_connected_cb')
+        # Break explicitly reference cycle
+        messages = None
 
 
 if __name__ == '__main__':