pitrou's patch changes PyFile_FromFd() behaviour for a text file 
opened with buffering=0:
  /* As a convenience, when buffering == 0 on a text file, we
     open the underlying binary stream in unbuffered mode and
     wrap it with a text stream in line-buffered mode. */

Why changing PyFile_FromFd() and not io.open() directly?

Note: I prefer Py_UnbufferedStdoutFlag=1 instead of 
Py_UnbufferedStdoutFlag++ (for -u command line option).

Except the minor comments, I like the patch (and it has unit 
tests!) ;-)