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/sudoku.py

@@ -9,9 +9,9 @@ function on the next column to see if it returns True. if yes, we
 have solved the puzzle. else, we backtrack and place another number
 in that cell and repeat this process.
 """
-from typing import List, Optional, Tuple
+from __future__ import annotations
 
-Matrix = List[List[int]]
+Matrix = list[list[int]]
 
 # assigning initial values to the grid
 initial_grid: Matrix = [
@@ -59,7 +59,7 @@ def is_safe(grid: Matrix, row: int, column: int, n: int) -> bool:
     return True
 
 
-def find_empty_location(grid: Matrix) -> Optional[Tuple[int, int]]:
+def find_empty_location(grid: Matrix) -> tuple[int, int] | None:
     """
     This function finds an empty location so that we can assign a number
     for that particular row and column.
@@ -71,7 +71,7 @@ def find_empty_location(grid: Matrix) -> Optional[Tuple[int, int]]:
     return None
 
 
-def sudoku(grid: Matrix) -> Optional[Matrix]:
+def sudoku(grid: Matrix) -> Matrix | None:
     """
     Takes a partially filled-in grid and attempts to assign values to
     all unassigned locations in such a way to meet the requirements