input: Write a function to convert a given string to its lowercase version. Return the lowercase string. If the input string is empty, return an empty string.
  assert str_to_lower("HELLO")=="hello"
  assert str_to_lower("World")=="world"
  assert str_to_lower("123ABCabc")=="123abcabc"
  assert str_to_lower("")==""
output: def str_to_lower(s):
      return s.lower()