Deepcopy should not cause a recursion error on the ProductAttributesC… (#4018)

* Deepcopy should not cause a recursion error on the ProductAttributesContainer

src/oscar/apps/catalogue/product_attributes.py

     """
 
     def __setstate__(self, state):
+        self.__dict__.setdefault("_product", None)
+        self.__dict__.setdefault("_initialized", False)
+        self.__dict__.setdefault("_dirty", set())
         self.__dict__ = state
 
     def __init__(self, product):

tests/unit/catalogue/test_product_attributes.py

+from copy import deepcopy
+
 from django.core.exceptions import ValidationError
 from django.test import TestCase
 
             "so it saved, even when the parent has the same value",
         )
 
+    def test_deepcopy(self):
+        "Deepcopy should not cause a recursion error"
+        deepcopy(self.product)
+        deepcopy(self.child_product)
+
 
 class ProductAttributeQuerysetTest(TestCase):
     fixtures = ["productattributes"]