input: Write a function to encrypt a message using the ROT13 cipher.
  assert encrypt('HELLO') == 'URYYB'
  assert encrypt('PYTHON') == 'CLGUBA'
  assert encrypt('ABCD EFGH') == 'NOPQ RSTU'
output: def encrypt(message):
      def rot13(char):
          if 'A' <= char <= 'Z':
              return chr((ord(char) - ord('A') + 13) % 26 + ord('A'))
          elif 'a' <= char <= 'z':
              return chr((ord(char) - ord('a') + 13) % 26 + ord('a'))
          else:
              return char
  
      return ''.join(rot13(char) for char in message)