@christian: Your patch doesn't work: if close() if called twice, the 
file is really closed :-/

Here is a new patch:
 - _FileIO.close() sets self->fd to -1 but don't show a warning anymore 
because the operation is valid: it does really close the _FileIO object
 - add two tests for two problems: read is now blocked if the file is 
closed, and the warning displayed by FileIO.__del__