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>

knapsack/knapsack.py

@@ -1,11 +1,10 @@
-from typing import List
-
 """ A naive recursive implementation of 0-1 Knapsack Problem
     https://en.wikipedia.org/wiki/Knapsack_problem
 """
+from __future__ import annotations
 
 
-def knapsack(capacity: int, weights: List[int], values: List[int], counter: int) -> int:
+def knapsack(capacity: int, weights: list[int], values: list[int], counter: int) -> int:
     """
     Returns the maximum value that can be put in a knapsack of a capacity cap,
     whereby each weight w has a specific value val.