shut down the pool's workers

        this method sets the :attr:`closing` attribute, and once all queued
        work has been completed it will set the :attr:`closed` attribute

def close(self):
        """shut down the pool's workers

        this method sets the :attr:`closing` attribute, and once all queued
        work has been completed it will set the :attr:`closed` attribute
        """
        self._closing = True
        for i in xrange(self.size):
            self.inq.put(_STOP)