input: def insertion_sort(arr: list) -> list:
      """
      Implement an insertion sort on a given list of integers.
      
      >>> insertion_sort([3, 2, 1])
      [1, 2, 3]
      >>> insertion_sort([5, 6, 4, 3, 8])
      [3, 4, 5, 6, 8]
      >>> insertion_sort([])
      []
      """
      for i in range(1, len(arr)):
          key = arr[i]
          j = i - 1
          while j >= 0 and key < arr[j]:
              arr[j + 1] = arr[j]
              j = j - 1
          arr[j + 1] = key
      return arr
output:     for i in range(1, len(arr)):
          key = arr[i]
          j = i - 1
          while j >= 0 and key < arr[j]:
              arr[j + 1] = arr[j]
              j = j - 1
          arr[j + 1] = key
      return arr