adding function declarations to hashtable.pyi

Author: realead

pandas/_libs/hashtable.pyi

@@ -228,3 +228,5 @@ def ismember(
     arr: np.ndarray,
     values: np.ndarray,
 ) -> np.ndarray: ...  # np.ndarray[bool]
+def object_hash(obj) -> int: ...
+def objects_are_equal(a, b) -> bool: ...