Remove circular reference check on item metadata hashing

Unless someone actively tries to break things by passing a crafted object, it should not generally be possible for there to be circular references. In addition, if someone were to do that, things would break in many other places, e.g. `MetadataPreparedRequest`, anyway. Guarding against a crash on hashing in that specific unusual scenario does not seem worthwhile and breaks when ujson is in use.

Fixes #540

internetarchive/item.py

@@ -131,8 +131,7 @@ def __hash__(self) -> int:
         without_excluded_keys = {
             k: v for k, v in self.item_metadata.items()
             if k not in self.EXCLUDED_ITEM_METADATA_KEYS}
-        return hash(json.dumps(without_excluded_keys,
-                               sort_keys=True, check_circular=False))  # type: ignore
+        return hash(json.dumps(without_excluded_keys, sort_keys=True))  # type: ignore
 
 
 class Item(BaseItem):