DeprecationWarning: Non-string usernames will no longer be supported in Requests 3.0.0. Please convert the object you've passed in (None) to a string or bytes object in the near future to avoid problems.

https://github.com/requests/requests/blob/f3fc892b901abe2c3605f2669af4f6469b896e88/requests/auth.py#L38-L45

Maybe Client._auth should return a function that does the same as HTTPBasicAuth.__call__?