add retries on mirrors.json/requests.json fetching

[bkmartinjr]

Our GHA's periodically fail due to network-level failures in fetching mirrors.json/requests.json, when access via the cellxgene-census R and/or Python packages. It is likely that other users see similar issues, and that they are caused by (inevitable & occasional) network instability.

Suggested fix: add a small number of retries, with appropriate backoff, to the fetch of release.json and mirrors.json

For Python, which uses the requests package, there is already built-in support that can be enabled - example here.

To do:

• [ ] Python package retries
• [ ] R package retries

[pablo-gar]

We'll monitor if caching has not fully solved this