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>

sorts/msd_radix_sort.py

@@ -4,10 +4,10 @@ It used the binary representation of the integers to sort
 them.
 https://en.wikipedia.org/wiki/Radix_sort
 """
-from typing import List
+from __future__ import annotations
 
 
-def msd_radix_sort(list_of_ints: List[int]) -> List[int]:
+def msd_radix_sort(list_of_ints: list[int]) -> list[int]:
     """
     Implementation of the MSD radix sort algorithm. Only works
     with positive integers
@@ -36,7 +36,7 @@ def msd_radix_sort(list_of_ints: List[int]) -> List[int]:
     return _msd_radix_sort(list_of_ints, most_bits)
 
 
-def _msd_radix_sort(list_of_ints: List[int], bit_position: int) -> List[int]:
+def _msd_radix_sort(list_of_ints: list[int], bit_position: int) -> list[int]:
     """
     Sort the given list based on the bit at bit_position. Numbers with a
     0 at that position will be at the start of the list, numbers with a
@@ -74,7 +74,7 @@ def _msd_radix_sort(list_of_ints: List[int], bit_position: int) -> List[int]:
     return res
 
 
-def msd_radix_sort_inplace(list_of_ints: List[int]):
+def msd_radix_sort_inplace(list_of_ints: list[int]):
     """
     Inplace implementation of the MSD radix sort algorithm.
     Sorts based on the binary representation of the integers.
@@ -109,7 +109,7 @@ def msd_radix_sort_inplace(list_of_ints: List[int]):
 
 
 def _msd_radix_sort_inplace(
-    list_of_ints: List[int], bit_position: int, begin_index: int, end_index: int
+    list_of_ints: list[int], bit_position: int, begin_index: int, end_index: int
 ):
     """
     Sort the given list based on the bit at bit_position. Numbers with a