input: Write a function to perform insertion sort on a given list.
  assert insertion_sort([5, 2, 9, 1, 5, 6]) == [1, 2, 5, 5, 6, 9]
  assert insertion_sort([0, -1, 3, 8, 7, 3]) == [-1, 0, 3, 3, 7, 8]
  assert insertion_sort([10, 20, 5, 15, 0, 25]) == [0, 5, 10, 15, 20, 25]
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