Migrate WhisperBack from python-gnupginterface to python-gnupg

Tails Greeter currently uses GnuPGInterface whose not maintained anymore. It should be ported to python-gnupg which now seems to include all the features we need and is packaged in jessie for python3.

Related issues

Is duplicate of #7755 (closed)

Original created by @alant on 7805 (Redmine)

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