title: cannot import name get_user_model
body: <p>I use django-registrations and while I add this code in my admin.py</p>
  
  <pre><code>   from django.contrib import admin
     from customer.models import Customer
     from .models import UserProfile
     from django.contrib.auth.admin import UserAdmin
     from django.contrib.auth import get_user_model
  
     class UserProfileInline(admin.StackedInline):
         model = UserProfile
         can_delete = False
  
     class UserProfileAdmin(UserAdmin):
         inlines=(UserProfileInline, )
  
     admin.site.unregister(get_user_model())
     admin.site.register(get_user_model(), UserProfileAdmin)
     admin.site.register(Customer)
  </code></pre>
  
  <p>I receive an error:</p>
  
  <pre><code>" cannot import name get_user_model "
  in admin.py
  </code></pre>
  
  <p>what am I doing wrong?</p>