Handle `Retry-After` response header on `HTTP 429 Too Many Requests` responses

[leplatrem]

Error https://mozilla.sentry.io/issues/4202640515/?project=6364263

Follow-up of #558 (comment)

this is interesting

@backoff.on_predicate(
    backoff.runtime,
    predicate=lambda r: r.status_code == 429,
    value=lambda r: int(r.headers.get("Retry-After")),
    jitter=None,
)
def get_url():
    return requests.get(url)