input: Write a function to convert a given string to a different case. The function should swap the case of each character in the string and return the new string.
  assert convert_string_case("this Is A TeST")=="THIS iS a tEst"
  assert convert_string_case("Hello World")=="hELLO wORLD"
  assert convert_string_case("123")=="123"
  assert convert_string_case("")==""
  assert convert_string_case("A b C d E")=="a B c D e"
output: def convert_string_case(s):
      return s.swapcase()