google.auth.default raises 404 when run from Travis with Python 3.7.

[tswast]

Failed to retrieve http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/default/?recursive=true from the Google Compute Enginemetadata service. Status: 404 Response

See: https://travis-ci.org/max-sixty/pandas-gbq/jobs/422752133 for googleapis/python-bigquery-pandas#206

I think this is related to the fact that Travis CI runs on GCE but it locks down access to APIs. Somehow google.auth.default() is not catching the 404.

/cc @max-sixty

[tseaver]

google.auth._default._get_gce_credentials and google.auth.compute_engine._metadata.ping each catch exceptions.TransportError from the request object, which is by default an instance of google.auth.transport._http_client.Request. Is pandas-gbq overriding that default?

[tswast]

No, pandas-gbq isn't overriding anything in google-auth. I'm not sure why this error seems to only show up in Python 3.7.

[tswast]

Looks like this error started popping up on other versions of Python, too. https://travis-ci.org/pydata/pandas-gbq/jobs/437226967

I think the issue is that google.auth.default() can throw a google.auth.exceptions.RefreshError on Travis and that pandas-gbq is only looking for DefaultCredentialsError. Shouldn't python-google-auth catch RefreshError and re-throw as DefaultCredentialsError in this case? (Or maybe skip checking metadata server in this case and move on to the next step?)