input: def str_to_lower(string: str) -> str:
      """
      Convert 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("mixedCASE123")
      'mixedcase123'
      """
      new_string = ""
      for char in string:
          new_string += char.lower()
      return new_string
output:     new_string = ""
      for char in string:
          new_string += char.lower()
      return new_string