[mypy] Add/fix type annotations for radix_sort(TheAlgorithms#4085)

Author: MatthewG25

Diff for: sorts/radix_sort.py

@@ -30,7 +30,7 @@ def radix_sort(list_of_ints: List[int]) -> List[int]:
     max_digit = max(list_of_ints)
     while placement <= max_digit:
         # declare and initialize empty buckets
-        buckets = [list() for _ in range(RADIX)]
+        buckets: List[list] = [list() for _ in range(RADIX)]
         # split list_of_ints between the buckets
         for i in list_of_ints:
             tmp = int((i / placement) % RADIX)