input: Write a function to shift each letter in a sentence to the next letter in the alphabet, keeping non-letter characters unchanged.
  assert alphabet_order("Hello, World!") == "Ifmmp, Xpsme!"
  assert alphabet_order("abcXYZ") == "bcdYZA"
  assert alphabet_order("Python 3.8") == "Qzuipo 3.8"
output: def alphabet_order(sentence):
      result = []
      for char in sentence:
          if 'a' <= char <= 'z':
              result.append(chr((ord(char) - ord('a') + 1) % 26 + ord('a')))
          elif 'A' <= char <= 'Z':
              result.append(chr((ord(char) - ord('A') + 1) % 26 + ord('A')))
          else:
              result.append(char)
      return ''.join(result)