input: Write a python function to convert a list of integers to a list of strings.
  assert to_string_list([1, 2, 3]) == ['1', '2', '3']
  assert to_string_list([10, 20, 30]) == ['10', '20', '30']
  assert to_string_list([5, 7, 9]) == ['5', '7', '9']
output: def to_string_list(int_list):
      return [str(i) for i in int_list]