input: Write a python function to perform insertion sort on a list of integers.
  assert insertion_sort([9, 4, 7, 2, 3])==[2, 3, 4, 7, 9]
  assert insertion_sort([5, 1, 4, 2, 8])==[1, 2, 4, 5, 8]
  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