Witness: fix items being modified by other slots (ArchipelagoMW#2161)

Author: blastron

worlds/witness/items.py

@@ -152,7 +152,7 @@ def get_mandatory_items(self) -> Dict[str, int]:
         """
         Returns the list of items that must be in the pool for the game to successfully generate.
         """
-        return self._mandatory_items
+        return self._mandatory_items.copy()
 
     def get_filler_items(self, quantity: int) -> Dict[str, int]:
         """