socket_real_close-5.patch: my own patch developed for the issue #4791 
(which is exactly the same problem) to fix SocketIO:
 - set self._sock=None on close
 - update the io reference count directly in close() (but also in 
destructor)
 - add a regression test

You may merge my patch with gps's patch (which has different tests).

See also issue #4853 to block all operations on closed socket in the 
low level API (socketmodule.c).