google.auth.exceptions.TransportError is not being retried

[nitishxp]

Hi,

Recently i faced an error can you please consider using this exception as a retry one also. Since i have faced this error in one of our production system

googleapis/python-storage#414

[tswast]

Based on the context from @busunkim96 in the linked issue, this is a reasonable request. We'll need a PR similar to #571 (retry ConnectionError by default)