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>

scheduling/round_robin.py

@@ -3,11 +3,12 @@ Round Robin is a scheduling algorithm.
 In Round Robin each process is assigned a fixed time slot in a cyclic way.
 https://en.wikipedia.org/wiki/Round-robin_scheduling
 """
+from __future__ import annotations
+
 from statistics import mean
-from typing import List
 
 
-def calculate_waiting_times(burst_times: List[int]) -> List[int]:
+def calculate_waiting_times(burst_times: list[int]) -> list[int]:
     """
     Calculate the waiting times of a list of processes that have a specified duration.
 
@@ -40,8 +41,8 @@ def calculate_waiting_times(burst_times: List[int]) -> List[int]:
 
 
 def calculate_turn_around_times(
-    burst_times: List[int], waiting_times: List[int]
-) -> List[int]:
+    burst_times: list[int], waiting_times: list[int]
+) -> list[int]:
     """
     >>> calculate_turn_around_times([1, 2, 3, 4], [0, 1, 3])
     [1, 3, 6]