Close socket on disconnect

As per issue altdesktop#137, sockets are not closed when the bus is disconnected.
This results in users receiving warnings about unclosed sockets.

This PR adds a call to the socket's `close` method during `disconnect`.

Closes altdesktop#137

dbus_next/message_bus.py

@@ -430,6 +430,7 @@ def disconnect(self):
         self._user_disconnect = True
         try:
             self._sock.shutdown(socket.SHUT_RDWR)
+            self._sock.close()
         except Exception:
             logging.warning('could not shut down socket', exc_info=True)