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 IS FUN") == "python programming is fun"
  assert str_to_lower("123@ABC") == "123@abc"
  assert str_to_lower("") == ""
  assert str_to_lower("ALL UPPERCASE") == "all lowercase"
output: def str_to_lower(string):
      return string.lower()