Deprecation of oauth2client dependency

[snoepkast]

According to the readme google/oauth2client is deprecated. They suggest switching to google-auth or oauthlib.

This probably means that this package and all of the docs should also be refactored into using these new packages?

[linar-jether]

+1
Any other usage docs for using google-auth or oauthlib with the api client libraries?

[xmedeko]

This lib still has a pip dependency to oauth2client and the code dependency, too, e.g. in the errors.py. Please, remove these dependencies and update the doc.

[amureki]

See comments here: #415

Seems like contributors are up for help with this one, if you have time and wish for it.