input: Write a python function that converts all lowercase letters in a string to uppercase and all uppercase letters to lowercase.
  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()