Prior to v2.17, GCS requests were always made unauthenticated.  With v2.17, GCS requests when using
the Python integration always use Google Application Default credentials, which can be set up with:

gcloud auth application-default login

However, if Google Application Default credentials were unavailable (e.g. because the user had not
set them up), GCS requests would fail even if the bucket allows unauthenticated reads.

With this change, reads to a bucket are first attempted without authenticated, and only if the
request fails is an authenticated request attempted.

Fixes #276.