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_081/sol1.py

@@ -22,7 +22,7 @@ def solution(filename: str = "matrix.txt") -> int:
     >>> solution()
     427337
     """
-    with open(os.path.join(os.path.dirname(__file__), filename), "r") as in_file:
+    with open(os.path.join(os.path.dirname(__file__), filename)) as in_file:
         data = in_file.read()
 
     grid = [[int(cell) for cell in row.split(",")] for row in data.strip().splitlines()]