input: Write a python function to transform each item in a list to a string.
  assert transform_to_string([1, 2, 3]) == ['1', '2', '3']
  assert transform_to_string([4, 5, 6]) == ['4', '5', '6']
  assert transform_to_string([7, 8, 9]) == ['7', '8', '9']
output: def transform_to_string(lst):
      return [str(item) for item in lst]