New patch:
 - Change _socket.fileno() behaviour: raise socket.error("I/O 
operation on closed socket") (instead of returning -1) if the socket 
is closed
 - Change SocketIO.fileno(): raise a socket.error("I/O operation on 
closed socket") if the file is closed, even if the socket is still 
open

TODO: Check all operations on closed _socket, socket or SocketIO and 
make sure that it raises also an error.

exakrun on IRC:
   What's the fascination with -1?
   Isn't this Python?  How about an exception? Or at least a different 
type... None, perhaps?
   Returning -1 is what you would do in a language like C where you 
*can't* do something nice.