input: Write a function that converts lowercase letters to uppercase and vice versa in a given string.
  assert swap_case('') == ''
  assert swap_case('Hello!') == 'hELLO!'
  assert swap_case('These violent delights have violent ends') == 'tHESE VIOLENT DELIGHTS HAVE VIOLENT ENDS'
output: def swap_case(s):
      return s.swapcase()