lookup_group returning Response objects

[ardichoke]

In a possible regression of #331 it seems lookup_group, and probably other functions as well, are returning Response objects when they get a 204 instead of None as the documentation claims it should. I have only verified this with lookup_group, as that happened to be the call where I encountered the problem.

[jeffwecan]

This is expected as part of the breaking changes in #537 (which tbqh wasn't handled all that well in terms of release, etc. 🙃).

So I'll intended to use this issue as reminder to update the documentation to match the behavior. 👍🏻