consider phasing in pgpy for some things

[hpk42]

Some things are quite annoying to do reliably with gpg/gpg2, like getting the openpgp4 fingerprint of ac-transmitted keydata. I suspect that also #24 will be annoying to do with gpg but not sure. So it's interesting to introduce pgpy for at least the fingerprint issue. It's likely muacrypt will retain gpg-support for dealing with keys managed by a user's gpg system key ring. But maybe incoming Autocrypt keys could be managed outside of gpg in general?

note that this also relates to #32 and the https://github.com/juga0/pyac effort where we can possibly re-use code and better understand the internal API.