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>

maths/average_mean.py

@@ -1,7 +1,7 @@
-from typing import List
+from __future__ import annotations
 
 
-def mean(nums: List) -> float:
+def mean(nums: list) -> float:
     """
     Find mean of a list of numbers.
     Wiki: https://en.wikipedia.org/wiki/Mean