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>

data_structures/stacks/stack.py

@@ -1,4 +1,4 @@
-from typing import List
+from __future__ import annotations
 
 
 class StackOverflowError(BaseException):
@@ -15,7 +15,7 @@ class Stack:
     """
 
     def __init__(self, limit: int = 10):
-        self.stack: List[int] = []
+        self.stack: list[int] = []
         self.limit = limit
 
     def __bool__(self) -> bool: