Add support for verifying v3 signature packets

We only support signature checking on v4+ keys, so hadn't any support
for v3 signature packets. However keys have been observed in the wild
where the key itself is a v4 key, but it's making v3 signatures (in
particular as a revocation signature for the key). While this seems odd
(anything that supports the key has to support v4, so why generate
anything lower?) it's probably too strict a check to have in place.