Migrate WhisperBack to Python 3

python-gnupginterface won’t be part of Jessie, so we need to migrate it to some other GnuPG Python bindings, such as python-gnupg (or isis’ fork, that has been ITP’d, or python3-gnupg.

Feature Branch: whisperback:feature/python3

Subtasks

• #7891 (closed)
• #7892 (closed)
• #7893 (closed)
• #7894 (closed)

Related issues

• Related to #9412 (closed)
• Related to #10577 (closed)
• Has duplicate #7805 (closed)
• Blocks #7756 (closed)
• Blocked by #8513 (closed)

Original created by @intrigeri on 7755 (Redmine)