input: Write a function to convert a string to its lowercase version.
  def str_to_lower(string):
      new_string = ""
      for char in string:
          new_string += char.lower()
      return new_string
  assert str_to_lower("HELLO WORLD") == "hello world"
  assert str_to_lower("Hello WORLD!") == "hello world!"
  assert str_to_lower("123 ABc!") == "123 abc!"
  assert str_to_lower("") == ""
  assert str_to_lower("Already Lower") == "already lower"
output: def str_to_lower(string):
      return string.lower()