title: PyNaCl failed to import SealedBox
body: <p>My code fails with this line:
  <code>from nacl.public import PrivateKey, SealedBox</code></p>
  
  <p>I got this error: ImportError: cannot import name 'SealedBox'</p>
  
  <p>I am using Python 3.6.  Anyone has any idea how this happen?</p>