Update zulipbot to deal with rate limits that cause it to miss events.

[laurynmm]

Zulipbot currently misses some events from GitHub due to rate-limiting, which often causes it to seem flaky for contributors (see links below). We should update zulipbot to avoid or better deal with rate limits to improve this.

Relevant conversations in CZO:

• https://chat.zulip.org/#narrow/channel/105-zulipbot/topic/zulipbot.20is.20confused.20.20.3F/near/1765402
• https://chat.zulip.org/#narrow/channel/9-issues/topic/zulipbot.20doesn't.20work/near/2068250