Replace oauth2client with google-auth

[alanyee]

Is your feature request related to a problem? Please describe.
oauth2client is deprecated. We should use its long-term replacement: https://github.com/googleapis/google-auth-library-python. Also this might fix #12

Describe the solution you'd like
https://developers.google.com/drive/api/v3/quickstart/python describes how https://github.com/googleapis/google-auth-library-python-oauthlib can replace oauth2client.

Describe alternatives you've considered
We can utilize having the credential file path be set via an environment variable via service_account.Credentials.from_service_account_file, or have the credential json string loaded explicitly via service_account.Credentials.from_service_account_info.

https://google-auth.readthedocs.io/en/latest/user-guide.html#service-account-private-key-files

Additional context
https://google-auth.readthedocs.io/en/latest/oauth2client-deprecation.html

This issue mostly affects how to migrate api.py. If we look at google_auth_oauthlib, for instance, credentials.invalid has been replaced by credentials.valid, client.flow_from_clientsecrets() is replaced by InstalledAppFlow.from_client_secrets_file(), tools.run_flow() is somewhat replaced by tools.run_console() and tools.run_local_server().