Apply Django upgrade for Django 2.2+

Author: vdboor

example/simpleshop/admin.py

@@ -2,10 +2,12 @@
 from simpleshop.models import Product, ProductCategory
 
 
+@admin.register(ProductCategory)
 class ProductCategoryAdmin(admin.ModelAdmin):
     prepopulated_fields = {"slug": ("title",)}
 
 
+@admin.register(Product)
 class ProductAdmin(admin.ModelAdmin):
     """
     A simple admin interface for the product administration.
@@ -16,5 +18,3 @@ class ProductAdmin(admin.ModelAdmin):
     prepopulated_fields = {"slug": ("title",)}
 
 
-admin.site.register(Product, ProductAdmin)
-admin.site.register(ProductCategory, ProductCategoryAdmin)