input: Write a python function to change the case of each character in a given string.
  assert change_case("This Is A Test String") == "tHIS iS a tEST sTRING"
  assert change_case("Hello World") == "hELLO wORLD"
  assert change_case("Python") == "pYTHON"
output: def change_case(s: str) -> str:
      return s.swapcase()