Sample does not provide working authentication

[jeffbryner]

python-samples/gmail/snippet/settings snippets/update_signature.py

Line 32 in 45699bb

creds, _ = google.auth.default()

Taking this sample and attempting to run it doesn't provide a working result.

Granted there is a note

    TODO(developer) - See https://developers.google.com/identity
    for guides on implementing OAuth2 for the application.

but that doesn't exactly help anyone figure out how to "make it go"

OK so scopes are missing? are they likely

    creds, _ = google.auth.default(scopes=['https://www.googleapis.com/auth/gmail.settings.basic', 'https://www.googleapis.com/auth/gmail.settings.sharing'])

Error returned when adding scopes results in:

An error occurred: <HttpError 403 when requesting https://gmail.googleapis.com/gmail/v1/users/me/settings/sendAs?alt=json returned "Request had insufficient authentication scopes.". Details: "[{'message': 'Insufficient Permission', 'domain': 'global', 'reason': 'insufficientPermissions'}]">

Any clues to how to make this sample work?