input: Write a python function to insert an item into a list at a specified index.
  assert insert_item([1, 3, 5, 7], 2, 1) == [1, 2, 3, 5, 7]
  assert insert_item([10, 20, 30, 40], 25, 2) == [10, 20, 25, 30, 40]
  assert insert_item(['a', 'b', 'c'], 'd', 3) == ['a', 'b', 'c', 'd']
output: def insert_item(lst, item, index):
      lst.insert(index, item)
      return lst