Move dead proxies to unchecked if a backoff timeout passes

def reanimate(self, _time=None):
        """ Move dead proxies to unchecked if a backoff timeout passes """
        n_reanimated = 0
        now = _time or time.time()
        for proxy in list(self.dead):
            state = self.proxies[proxy]
            assert state.next_check is not None
            if state.next_check <= now:
                self.dead.remove(proxy)
                self.unchecked.add(proxy)
                n_reanimated += 1
        return n_reanimated