I am reasonably certain that GitHub Enterprise functionality in the 3.0.0RC1 / dev branch is currently completely broken.
I don't have a GitHub enterprise account, so I can't test this functionality at all. If you do, I would really appreciate some help in understanding some things:
1) What do you need from your github enterprise account (in terms of AutoPkg's capabilities)?
2) When you need/want to access a private repo hosted on your github enterprise, how do you currently do it now?
3) What should AutoPkg do if the auth fails?
4) What would you lose/no longer be able to do if AutoPkg didn't support github enterprise access at all?