input: Write a function to convert a given string to its lowercase version. Return the lowercase version of the string. If the input string is empty, return an empty string as well.
  assert str_to_lower("Hello, World!")=="hello, world!"
  assert str_to_lower("PYTHON")=="python"
  assert str_to_lower("123abc")=="123abc"
  assert str_to_lower("")==""
output: def str_to_lower(s):
      return s.lower()