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/all_subsequences.py

@@ -5,15 +5,17 @@ of the given sequence. We use backtracking to solve this problem.
 Time complexity: O(2^n),
 where n denotes the length of the given sequence.
 """
-from typing import Any, List
+from __future__ import annotations
+
+from typing import Any
 
 
-def generate_all_subsequences(sequence: List[Any]) -> None:
+def generate_all_subsequences(sequence: list[Any]) -> None:
     create_state_space_tree(sequence, [], 0)
 
 
 def create_state_space_tree(
-    sequence: List[Any], current_subsequence: List[Any], index: int
+    sequence: list[Any], current_subsequence: list[Any], index: int
 ) -> None:
     """
     Creates a state space tree to iterate through each branch using DFS.
@@ -32,7 +34,7 @@ def create_state_space_tree(
 
 
 if __name__ == "__main__":
-    seq: List[Any] = [3, 1, 2, 4]
+    seq: list[Any] = [3, 1, 2, 4]
     generate_all_subsequences(seq)
 
     seq.clear()