input: Write a function that converts all lowercase letters in a string to uppercase and all uppercase letters to lowercase.
  assert switch_case('') == ''
  assert switch_case('Python!') == 'pYTHON!'
  assert switch_case('What We do in the Shadows') == 'wHAT wE DO IN THE sHADOWS'
output: def switch_case(s):
      return s.swapcase()