github3.null is gone

[bkruger99]

According to this commit:
sigmavirus24/github3.py#670

Looks like NullObject has been removed.

Will dig into it more and see, I might need to add some functionality in the process.

[jlevy]

Have not looked but perhaps we should pin the version of github3 this depends on for now, or do an upgrade to a later version officially. PRs welcome!