Update, also change from pname = "package-unstable"; version = "YYYY-MM-DD"; to pname = "package"; version = "unstable-YYYY-MM-DD"; This allows $pname as $repo in fetchFromGitHub and also seems to be the more common way in nixpkgs.
I initially looked at the wrong places for this, where it's actually stated in setup.py. I'm not quite a python guy.