Oh, by design, NetworkConnectionNoServer.test_create_connection() has a race condition:

        port = socket_helper.find_unused_port()
        with self.assertRaises(OSError) as cm:
            socket.create_connection((HOST, port))

If another process starts to listen to the "unused" port between find_unused_port() call and the create_connection() call, create_connection() succeed and the test fails.

Maybe I should just add a comment mentioning the race condition. Or we should find a more reliable way to get an error.