input: Write a function to convert a given 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 PROGRAMMING")=="python programming"
  assert str_to_lower("12345")=="12345"
  assert str_to_lower("")==""
  assert str_to_lower("AbCdEf")=="abcdef"
output: def str_to_lower(string):
      return string.lower()