The `expires_in` attribute of `AuthClient` is never set

https://github.com/intuit/oauth-pythonclient/blob/058eedf8f651a8af207e6f6d4a2ebf05e906f7b7/intuitlib/client.py#L67

This value never appears to be updated anywhere in the project. So no matter when this variable is referenced--it will always be `None`. One would imagine that this would need to be updated at some point, right? Of course, I could be missing something here as well.