Pyupgrade to Python 3.9 (#4718)

* Pyupgrade to Python 3.9

* updating DIRECTORY.md

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>

other/lru_cache.py

@@ -1,4 +1,6 @@
-from typing import Callable, Optional
+from __future__ import annotations
+
+from typing import Callable
 
 
 class DoubleLinkedListNode:
@@ -125,7 +127,7 @@ class LRUCache:
 
         return key in self.cache
 
-    def get(self, key: int) -> Optional[int]:
+    def get(self, key: int) -> int | None:
         """
         Returns the value for the input key and updates the Double Linked List. Returns
         None if key is not present in cache