Check for inactive workers and remove their references.

def _clean(self):
        """Check for inactive workers and remove their references."""
        if self._workers:
            for url in self._workers.copy():
                w = self._workers[url]
                if w.is_finished():
                    self._workers.pop(url)
                    self._paths.pop(url)
                    if url in self._get_requests:
                        self._get_requests.pop(url)

        else:
            self._timer.stop()