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