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("Python PROGramming123")=="python programming123"
  assert str_to_lower("ALL CAPS")=="all caps"
  assert str_to_lower("")==""
  assert str_to_lower("a1B2c3")=="a1b2c3"
output: def str_to_lower(string):
      return string.lower()