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