feat: retry google.auth TransportError and requests ConnectionError

[parthea]

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Fixes#176 🦕

[busunkim96]

Is there a way for a bare requests.exception to make it to this code? I was under the impression the auth library would always wrap it.

Otherwise LGTM.

[parthea]

@busunkim96 Here is an example of requests.exception in the stack trace in python-bigquery via googleapis/python-bigquery#434.