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>

project_euler/problem_036/sol1.py

@@ -14,11 +14,10 @@ base 10 and base 2.
 (Please note that the palindromic number, in either base, may not include
 leading zeros.)
 """
-
-from typing import Union
+from __future__ import annotations
 
 
-def is_palindrome(n: Union[int, str]) -> bool:
+def is_palindrome(n: int | str) -> bool:
     """
     Return true if the input n is a palindrome.
     Otherwise return false. n can be an integer or a string.