input: Write a python function to convert the case of each character in a string from upper to lower or lower to upper.
  assert convert_string_case("this Is A TeST") == "THIS iS a tEst"
  assert convert_string_case("Hello World!") == "hELLO wORLD!"
  assert convert_string_case("Python3.8") == "pYTHON3.8"
output: def convert_string_case(s):
      return s.swapcase()