fix: Different hash for same dictionaries problem (#32)

When merging unhashable types, the hash is calculated based on `str(obj)`. The problem appears when the hash is calculated for two dictionaries like
`a = {'foo': 1, 'bar': 2}` and `b = {'bar': 2, 'foo': 1}.`

In this case, the calculated hashes obviously don't match, though from any other perspective, the dictionaries have no difference.

deepmerge/extended_set.py

@@ -22,6 +22,9 @@ def _insert(self, element: Any) -> None:
     def _hash_element(self, element: Any) -> int:
         if getattr(element, "__hash__") is not None:
             return hash(element)
+        elif isinstance(element, dict):
+            sorted_keys = sorted(element.keys())
+            return hash(",".join([f"{key}:{element[key]}" for key in sorted_keys]))
         return hash(str(element))
 
     def __contains__(self, obj: Any) -> bool: