Has added tests for the promotion functions for Shufersal and CoOp. Also added minor design changes in promotion.py and item.py

item.py

@@ -3,11 +3,11 @@ class Item:
     A class representing a product in some supermarket.
     """
 
-    def __init__(self, name: str, price: float, manufacturer: str, code: int):
+    def __init__(self, name: str, price: float, manufacturer: str, code: str):
         self.name: str = name
         self.price: float = price
         self.manufacturer: str = manufacturer
-        self.code: int = code
+        self.code: str = code
 
     def __repr__(self):
         return str((self.name, self.price, self.manufacturer, self.code))