input: Write a python function to sort a list using insertion sort.
  assert insertion_sort([4, 3, 2, 10, 12, 1, 5, 6]) == [1, 2, 3, 4, 5, 6, 10, 12]
  assert insertion_sort([64, 34, 25, 12, 22, 11, 90]) == [11, 12, 22, 25, 34, 64, 90]
  assert insertion_sort([12, 11, 13, 5, 6]) == [5, 6, 11, 12, 13]
output: def insertion_sort(arr):
      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 -= 1
          arr[j + 1] = key
      return arr