github.lister: Use Retry-After header when rate limit reached

Following the documentation:

• [1] https://developer.github.com/v3/guides/best-practices-for-integrators/#dealing-with-abuse-rate-limits

This does not deal with the concurrency aspect. For that part, the goal is to deploy the github lister on 1 dedicated worker for now (done puppet side and deployed, worker01 got elected).

Related to #2170

Test Plan

tox

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Migrated from D2545 (view on Phabricator)