Add return type hint

Author: Hardvan

data_structures/linked_list/merge_sort_linked_list.py

@@ -15,7 +15,7 @@ class Node:
     1 2 3 4
     """
 
-    def __init__(self, data: int):
+    def __init__(self, data: int) -> None:
         self.data = data
         self.next: Node | None = None