[Project Euler] Fix code style for multiple problems (#3094)

* Fix code style for Project Euler problems:

- 13, 17, 21
- Default args
- Type hints
- File path

* Fix code style for multiple problems

* Made suggested changes

project_euler/problem_44/sol1.py

@@ -1,4 +1,6 @@
 """
+Problem 44: https://projecteuler.net/problem=44
+
 Pentagonal numbers are generated by the formula, Pn=n(3n−1)/2. The first ten
 pentagonal numbers are:
 1, 5, 12, 22, 35, 51, 70, 92, 117, 145, ...
@@ -24,11 +26,11 @@ def is_pentagonal(n: int) -> bool:
     return ((1 + root) / 6) % 1 == 0
 
 
-def compute_num(limit: int = 5000) -> int:
+def solution(limit: int = 5000) -> int:
     """
     Returns the minimum difference of two pentagonal numbers P1 and P2 such that
     P1 + P2 is pentagonal and P2 - P1 is pentagonal.
-    >>> compute_num(5000)
+    >>> solution(5000)
     5482660
     """
     pentagonal_nums = [(i * (3 * i - 1)) // 2 for i in range(1, limit)]
@@ -42,4 +44,4 @@ def compute_num(limit: int = 5000) -> int:
 
 
 if __name__ == "__main__":
-    print(f"{compute_num() = }")
+    print(f"{solution() = }")