Missing core models from admin after adding adminplus

Hi,

Just spent today integrating adminplus into our Django 1.8.7 site, found an issue where 3 of our admin screens went missing after the upgrade.Turns out they are all core models (Groups, Users and Sites) and these are all added by an autodiscover that django runs before it loads the urls file.
See traceback for when the first one is added:

Saw this in a previous issue and it has solved the issue for now although I am hoping someone smarter than I can help us solve it better:

import copy
from django.contrib import admin
from django.conf.urls import patterns, include, url

from adminplus.sites import AdminSitePlus

old = admin.site
admin.site = AdminSitePlus()
admin.site._registry = copy.copy(old._registry)

Start by examining Django admin autodiscover and the adminplus.sites.AdminSitePlus entry point, using the reported traceback to follow registration before URL loading. Compare the copied admin.site._registry workaround with the normal core-model registration flow. Done means Groups, Users, and Sites remain available in the admin without that workaround.