Improve forward compatibility

[dkg]

These tests are pulled from the OpenPGP Interoperability Test Suite, in particular from those tests tagged "forward-compat":

https://tests.sequoia-pgp.org/?q=forward-compat

These failures make it difficult to use PGPy in an OpenPGP ecosystem that can evolve, because PGPy will complain about OpenPGP objects that contain something it doesn't understand, even though the rest of the message is otherwise comprehensible and usable.

See Justus Winter's message to openpgp@ietf.org describing his concerns:

https://mailarchive.ietf.org/arch/msg/openpgp/QUiEKx3PQeJOXnkcvvnuHpv739M

I've selected specific examples that PGPy is known to currently fail with.

---

This series should make all the new tests pass, which should also improve the score of PGPy on the interop test suite.

[dkg]

These changes also include some (hopefully positive) side effects:

• remove the dependency on pyasn1, since arbitrary ASN.1 parsing isn't needed -- if the ECC curve is unknown, the point is unknown as well.
• clean up dependency on the cryptography library, closing #402 (see e015068b5abf8a52edc77ba45866b9b05a067b9d)
• introduce the beginnings of some type annotations
• add a PGPSignatures object which can be used to handle a detached OpenPGP signature object that contains more than one signature, by analogy with an inline-signed message that has more than one signature.