passphrase_cb is deprecated
    
    https://bugs.gnupg.org/gnupg/issue767 indicates that
    gpgme_set_passphrase_cb is a deprecated corner of the API and that
    developers using gpgme should really rely on the gpg-agent to handle
    this stuff.  This should actually simplify things for most
    installations -- just strip out all passphrase handling from your
    application entirely, relying on gpg to figure out how to find the
    agent, and relying on the agent figuring out how to prompt the user
    (if necessary).
    
    However, if a developer really wants to use the passphrase callback
    approach, they'll have to use loopback pinentry.  This sets up the
    test suite to be able to make those tests.