[feature request] Use get_serving_url_async

[asktree]

Thanks for this repo, I use it. However, I'm retrieving many thousands of serving urls and it would be more efficient to use the get_serving_url_async method provided by the package. I would implement it myself but I don't [want to] understand how to use async in Python 2.

Cheers.

[zvictor]

@asktree thanks for the interest! Your suggestion seems to be a good improvement to this project and I would merge a PR for that if you send it to us.

However, to be honest, we don't plan to invest much more time on this project ourselves any longer due to albertcht/python-gcs-image#3.

Based on your interest I decided to add disclaimers explaining it to new users and pointing to alternative solutions in our README.