Allow auth to be None. Fix #773

[lepture]

This fixed #773

cc @burnash I'd like to suggest using Authlib instead of google.auth. With Authlib, you just need to add very little code for the auth part in gspread. Check out: https://blog.authlib.org/2018/authlib-for-gspread

[burnash]

@lepture thank you very much for the fix and for Authlib.

Authlib is a great option however I think I'll stick with google.auth for now. Initially, I thought that decoupling gspread from oauth handlers was a good idea, however, after #637 and #711 the integration with google.auth has been working pretty well so far. Also allowed to easily implement nice shortcuts for auth: gspread.oauth() and gspread.service_account().

[ghost]

Thank you for the fix @lepture. When would be the next release? Would really like to have this in the official package.