Implement Attested Certifications

This makes the No-modify flag for Key Server Preferences actionable,
by allowing the primary key holder the ability to indicate which
third-party certifications are acceptable for redistribution.

See https://gitlab.com/openpgp-wg/rfc4880bis/merge_requests/20 for
more details.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>

tests/test_01_packetfields.py

@@ -108,6 +108,7 @@ _sspclasses = {
     # 0x1f: 'Target',  ##TODO: obtain one of these ##TODO: parse this, then uncomment
     0x20: 'EmbeddedSignature',
     0x21: 'IssuerFingerprint',
+    0x25: 'AttestedCertifications',
     # 0x64-0x6e: Private or Experimental
     0x64: 'Opaque',
     0x65: 'Opaque',