http://www.python.org/dev/peps/pep-0433/#close-file-descriptors-after-fork

I believe in Python 3.3+, the CLOSE_ON_EXEC flag is set by default.  However, since we're only forking without exec (IIRC), this file descriptor will stay open.  It looks as though they're maybe planning on making some way to easily close file descriptors with this flag using an atfork module, but for now, it should work (although admittedly, I haven't tested it).