Rename Project Euler directories and other dependent changes (#3300)

* Rename all Project Euler directories:

Reason:
The change was done to maintain consistency throughout the directory
and to keep all directories in sorted order.

Due to the above change, some config files had to be modified:
'problem_22` -> `problem_022`

* Update scripts to pad zeroes in PE directories

project_euler/problem_021/sol1.py

@@ -0,0 +1,54 @@
+"""
+Amicable Numbers
+Problem 21
+
+Let d(n) be defined as the sum of proper divisors of n (numbers less than n
+which divide evenly into n).
+If d(a) = b and d(b) = a, where a ≠ b, then a and b are an amicable pair and
+each of a and b are called amicable numbers.
+
+For example, the proper divisors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55
+and 110; therefore d(220) = 284. The proper divisors of 284 are 1, 2, 4, 71 and
+142; so d(284) = 220.
+
+Evaluate the sum of all the amicable numbers under 10000.
+"""
+from math import sqrt
+
+
+def sum_of_divisors(n: int) -> int:
+    total = 0
+    for i in range(1, int(sqrt(n) + 1)):
+        if n % i == 0 and i != sqrt(n):
+            total += i + n // i
+        elif i == sqrt(n):
+            total += i
+    return total - n
+
+
+def solution(n: int = 10000) -> int:
+    """Returns the sum of all the amicable numbers under n.
+
+    >>> solution(10000)
+    31626
+    >>> solution(5000)
+    8442
+    >>> solution(1000)
+    504
+    >>> solution(100)
+    0
+    >>> solution(50)
+    0
+    """
+    total = sum(
+        [
+            i
+            for i in range(1, n)
+            if sum_of_divisors(sum_of_divisors(i)) == i and sum_of_divisors(i) != i
+        ]
+    )
+    return total
+
+
+if __name__ == "__main__":
+    print(solution(int(str(input()).strip())))