input: def str_to_lower(string: str) -> str:
      """ Converts the given string to its lowercase version.
      >>> str_to_lower("Hello World")
      "hello world"
      >>> str_to_lower("PYTHON")
      "python"
      >>> str_to_lower("cAsE iNsEnSiTiViTy")
      "case insensitivity"
      """
output:     return string.lower()