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_020/sol2.py

@@ -28,7 +28,7 @@ def solution(num: int = 100) -> int:
     >>> solution(1)
     1
     """
-    return sum([int(x) for x in str(factorial(num))])
+    return sum(int(x) for x in str(factorial(num)))
 
 
 if __name__ == "__main__":