input: 
  def str_to_lower(string: str) -> str:
      """ Converts a given string to its lowercase version.
      >>> str_to_lower("HELLO WORLD")
      'hello world'
      >>> str_to_lower("ThIs Is A TeSt")
      'this is a test'
      >>> str_to_lower("123ABC")
      '123abc'
      """
  
output:     return string.lower()