Try to use GitHub PAT when fetching forecast dates; and check response status for rate limit

[nmdefries]

Followup to #647. Because dates are now being fetched using the GitHub API, scripts that request a lot of dates (e.g. forecast eval pipeline) can run into rate limits. When the API is rate-limited, get_covidhub_forecast_dates raises a non-informative warning and returns empty dates, causing downstream failures that are hard to debug.

Look for a GITHUB_PAT environment variable first, then GITHUB_TOKEN if *_PAT doesn't exist. If either is found, use it to authenticate the GET requests (using this header format). Response status codes are checked and if a 403 status is returned, the function exits with an error. It doesn't make sense to warn and return anyway because the results are empty; or to retry with backoff because the rate limit resets after an hour.

Note: I used the API's message as the error message because I'm not sure a 403-status is unique to reaching the rate limit and I want the actual error to be clear, and checking the text of the message wouldn't be robust.

[dshemetov]

looks great, thanks for taking care of this!

i checked locally that both auth and unauth requests work and that the error comes through well 👍