input: Write a python function to shift each letter in a string to the next letter in the alphabet.
  assert shift_words("hello world") == "ifmmp xpsme"
  assert shift_words("abc xyz") == "bcd yza"
  assert shift_words("Python 3.8") == "Qzuipo 3.8"
output: def shift_words(s):
      def shift_char(c):
          if 'a' <= c <= 'z':
              return chr(((ord(c) - ord('a') + 1) % 26) + ord('a'))
          elif 'A' <= c <= 'Z':
              return chr(((ord(c) - ord('A') + 1) % 26) + ord('A'))
          else:
              return c
  
      return ''.join(shift_char(c) for c in s)