input: Write a python function to convert all uppercase letters to lowercase and all lowercase letters to uppercase in a given string.
  assert swap_case("HELLO WORLD") == "hello world"
  assert swap_case("Python Is Fun") == "pYTHON iS fUN"
  assert swap_case("123ABCabc!") == "123abcABC!"
output: def swap_case(s: str) -> str:
      return s.swapcase()