Tighten up psf/black and flake8 (#2024)

* Tighten up psf/black and flake8

* Fix some tests

* Fix some E741

* Fix some E741

* updating DIRECTORY.md

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>

dynamic_programming/factorial.py

@@ -26,9 +26,9 @@ def factorial(num):
 
 # factorial of num
 # uncomment the following to see how recalculations are avoided
-##result=[-1]*10
-##result[0]=result[1]=1
-##print(factorial(5))
+# result=[-1]*10
+# result[0]=result[1]=1
+# print(factorial(5))
 # print(factorial(3))
 # print(factorial(7))