Move more SignatureSubpacket logic to actual class
Move more SignatureSubpacket logic to actual class

We had the lookup methods on the signature packet itself rather than the
subclass. Move it into the subtype class instead, and lookup the type
name lazily.

Signed-off-by: Dan McGee <a6e5737275ff1276377ee261739f3ee963671241@gmail.com>

toofishes/python-pgpdump