input: Write a function to convert a given string to its lowercase version. Return the lowercase string.
  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")=="python"
  assert str_to_lower("123abcXYZ")=="123abcxyz"
  assert str_to_lower("")==""
output: def str_to_lower(string):
      return string.lower()