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>

data_structures/stacks/evaluate_postfix_notations.py

@@ -1,5 +1,3 @@
-from typing import Any, List
-
 """
 The Reverse Polish Nation also known as Polish postfix notation
 or simply postfix notation.
@@ -8,6 +6,9 @@ Classic examples of simple stack implementations
 Valid operators are +, -, *, /.
 Each operand may be an integer or another expression.
 """
+from __future__ import annotations
+
+from typing import Any
 
 
 def evaluate_postfix(postfix_notation: list) -> int:
@@ -23,7 +24,7 @@ def evaluate_postfix(postfix_notation: list) -> int:
         return 0
 
     operations = {"+", "-", "*", "/"}
-    stack: List[Any] = []
+    stack: list[Any] = []
 
     for token in postfix_notation:
         if token in operations:

data_structures/stacks/linked_stack.py

@@ -1,5 +1,7 @@
 """ A Stack using a linked list like structure """
-from typing import Any, Optional
+from __future__ import annotations
+
+from typing import Any
 
 
 class Node:
@@ -42,7 +44,7 @@ class LinkedStack:
     """
 
     def __init__(self) -> None:
-        self.top: Optional[Node] = None
+        self.top: Node | None = None
 
     def __iter__(self):
         node = self.top

data_structures/stacks/stack.py

@@ -1,4 +1,4 @@
-from typing import List
+from __future__ import annotations
 
 
 class StackOverflowError(BaseException):
@@ -15,7 +15,7 @@ class Stack:
     """
 
     def __init__(self, limit: int = 10):
-        self.stack: List[int] = []
+        self.stack: list[int] = []
         self.limit = limit
 
     def __bool__(self) -> bool: