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>

backtracking/all_combinations.py

@@ -3,16 +3,16 @@
         numbers out of 1 ... n. We use backtracking to solve this problem.
         Time complexity: O(C(n,k)) which is O(n choose k) = O((n!/(k! * (n - k)!)))
 """
-from typing import List
+from __future__ import annotations
 
 
-def generate_all_combinations(n: int, k: int) -> List[List[int]]:
+def generate_all_combinations(n: int, k: int) -> list[list[int]]:
     """
     >>> generate_all_combinations(n=4, k=2)
     [[1, 2], [1, 3], [1, 4], [2, 3], [2, 4], [3, 4]]
     """
 
-    result: List[List[int]] = []
+    result: list[list[int]] = []
     create_all_state(1, n, k, [], result)
     return result
 
@@ -21,8 +21,8 @@ def create_all_state(
     increment: int,
     total_number: int,
     level: int,
-    current_list: List[int],
-    total_list: List[List[int]],
+    current_list: list[int],
+    total_list: list[list[int]],
 ) -> None:
     if level == 0:
         total_list.append(current_list[:])
@@ -34,7 +34,7 @@ def create_all_state(
         current_list.pop()
 
 
-def print_all_state(total_list: List[List[int]]) -> None:
+def print_all_state(total_list: list[list[int]]) -> None:
     for i in total_list:
         print(*i)