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/n_queens_math.py

@@ -75,14 +75,14 @@ Applying this two formulas we can check if a queen in some position is being att
 for another one or vice versa.
 
 """
-from typing import List
+from __future__ import annotations
 
 
 def depth_first_search(
-    possible_board: List[int],
-    diagonal_right_collisions: List[int],
-    diagonal_left_collisions: List[int],
-    boards: List[List[str]],
+    possible_board: list[int],
+    diagonal_right_collisions: list[int],
+    diagonal_left_collisions: list[int],
+    boards: list[list[str]],
     n: int,
 ) -> None:
     """
@@ -139,7 +139,7 @@ def depth_first_search(
 
 
 def n_queens_solution(n: int) -> None:
-    boards: List[List[str]] = []
+    boards: list[list[str]] = []
     depth_first_search([], [], [], boards, n)
 
     # Print all the boards