labels: retry request for PR files if GitHub request fails

[phillipj]

Trying to get around some intermittent GitHub API 404 failures, probably due to a race condition where the API hasn't yet got to update its PR cache or something similar.

Refs #88 (comment)

[mscdex]

IMHO we shouldn't pull in async for something that could be accomplished with setTimeout() and a counter.

[phillipj]

See your point in async being overkill for one function. Though writing something similar ourself should ideally include tests for that particular code, as well as maintaining it over time. Since this is strictly backend and not frontend where we'd consider JS-bundle sizes, I personally see the benefits greater than the downsides in including a battle tested package like async.

Got any thoughts on where the threshold should be for knowing when to use a 3rd party package or write it ourself?

P.S. I really had two async use cases in mind when open this PR -> #128 / 458fdc6#diff-3b3f42a9fe8032799be4775b56e049dfR14 as well.