(internal) executes when last output piper raises ``StopIteration``.

def _finish(self, ispausing):
        """
        (internal) executes when last output piper raises ``StopIteration``.
        """
        if ispausing:
            self.log.debug('%s paused' % repr(self))
        else:
            self._finished.set()
            self.log.debug('%s finished' % repr(self))