input: def str_to_lower(string: str) -> str:
      """Convert a string to its lowercase version.
      >>> str_to_lower("HELLO World")
      'hello world'
      >>> str_to_lower("Python 3.9")
      'python 3.9'
      """
      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