Price is not a relation and cannot select_related

This field is ignored in 1.9, however in 1.10+ it is an error.
As this is a no-op in 1.9, removal keeps functionality while
extending compatability going forward.

For full details please see Django Ticket 10414 at:
https://code.djangoproject.com/ticket/10414

registrasion/controllers/cart.py

@@ -445,7 +445,7 @@ class CartController(object):
         # Order the products such that the most expensive ones are
         # processed first.
         product_items = self.cart.productitem_set.all().select_related(
-            "product", "product__category", "product__price"
+            "product", "product__category"
         ).order_by("-product__price")
 
         products = [i.product for i in product_items]