doc: crypto.rst — GPGME HOWTO update

* Added links to the new GPGME Python Bindings HOWTO.
* Added links to sample scripts written in Python 3.
* Updated the encryption and decryption example code to use the
  current best practices instead of the underlying, low level
  op_encrypt method (the best practices are easier and even more
  pythonic).

Signed-off-by: Ben McGinnes <ben@adversary.org>