input: Write a python function to perform insertion sort on a list of integers.
  assert insertion_sort([1, 3, 5, 2, 4]) == [1, 2, 3, 4, 5]
  assert insertion_sort([10, 7, 3, 1, 9]) == [1, 3, 7, 9, 10]
  assert insertion_sort([4, 5, 2, 9, 6]) == [2, 4, 5, 6, 9]
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