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_006/sol3.py

@@ -33,7 +33,7 @@ def solution(n: int = 100) -> int:
     1582700
     """
 
-    sum_of_squares = sum([i * i for i in range(1, n + 1)])
+    sum_of_squares = sum(i * i for i in range(1, n + 1))
     square_of_sum = int(math.pow(sum(range(1, n + 1)), 2))
     return square_of_sum - sum_of_squares