Updated problem_06 in Project Euler (#2439)

* * rename variable
* fix type hint
* fix doctest

* added test function

* fixed import error

* fixup! Format Python code with psf/black push

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

project_euler/problem_06/sol4.py

@@ -15,7 +15,7 @@ numbers and the square of the sum.
 """
 
 
-def solution(n):
+def solution(n: int) -> int:
     """Returns the difference between the sum of the squares of the first n
     natural numbers and the square of the sum.
 
@@ -36,4 +36,7 @@ def solution(n):
 
 
 if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()
     print(solution(int(input("Enter a number: ").strip())))