Update dependencies & remove oath2client

[Mr0grog]

oauth2client, which we use in some parts of the code here, has been deprecated: https://google-auth.readthedocs.io/en/latest/oauth2client-deprecation.html#post-deprecation-support

I’m not sure whether that means changes on the remote end may eventually cause it to stop working, but we should probably replace our usage of it with something new.

Several other dependencies (e.g. google-api-python-client and google-auth) are also out of date, and the latest versions of them appear to also no longer use oauth2client.