Mark a proxy as dead

def mark_dead(self, proxy, _time=None):
        """ Mark a proxy as dead """
        if proxy not in self.proxies:
            logger.warn("Proxy <%s> was not found in proxies list" % proxy)
            return

        if proxy in self.good:
            logger.debug("GOOD proxy became DEAD: <%s>" % proxy)
        else:
            logger.debug("Proxy <%s> is DEAD" % proxy)

        self.unchecked.discard(proxy)
        self.good.discard(proxy)
        self.dead.add(proxy)

        now = _time or time.time()
        state = self.proxies[proxy]
        state.backoff_time = self.backoff(state.failed_attempts)
        state.next_check = now + state.backoff_time
        state.failed_attempts += 1