verifySignature lets a key that has expired since verify a signature,
because a manifest outlives its key, but it checked the key only as of
now. It accepted a signature dated after its key expired, after the
key's self-signature or the signing subkey's binding signature expired,
or before the key was made. It now verifies a second time with the
library's clock set to the signature's creation time and refuses the
signature if the key or subkey was not valid then. docs/FORMAT.md
states the rule.
Judgement call: the tests make the refused signatures with the
library's packet API, because openpgp.DetachSign will not sign with a
key that is not valid at its clock's time.
Model: opus-5-5
sneak