input: Write a function to convert a given string to its lowercase version.
  assert str_to_lower("Hello World")=="hello world"
  assert str_to_lower("PYTHON")=="python"
  assert str_to_lower("")==""
  assert str_to_lower("123ABC")=="123abc"
  assert str_to_lower("!@#$%^&*")=="!@#$%^&*"
output: def str_to_lower(s):
      return s.lower()