Switch from python-gnupg to gnupg, update calls

* "gnupg" is a fork of python-gnupg which seems more actively maintained and
  more responsive to bug reports than python-gnupg
* Updates to the latest version of gnupg and updates gnupg calls to
  preserve semantics where the API changed

Stanford isn't using this in production (yet), but I have manually tested and everything seems to be in order. It's a (mostly) drop-in changeover.