input: Write a function to convert a 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("Python3")=="python3"
  assert str_to_lower("123ABC")=="123abc"
  assert str_to_lower("")==""
  assert str_to_lower("ALREADY lowerCASE")=="already lowercase"
output: def str_to_lower(string):
      return string.lower()