(transferred from in-toto/in-toto#263)

Description of issue or feature request:
in-toto/in-toto#257 adds gpg self-signature verification support, for signatures types 0x10-0x13 (certifications) and 0x18 (subkey binding signature). The added infrastructure may be used to also consider other signature types such as,

• 0x20: Key revocation signature
• 0x28: Subkey revocation signature
• 0x30: Certification revocation signature

See RFC4880 5.2.1. Signature Types and 12.1. Key Structures for details about the signatures types and where in the key bundle they occur.

Current behavior:
Revocation type gpg signatures are ignored.

Expected behavior:
Handle revocation type gpg signatures, e.g. ignore revoked keys or certificates and/or warn user about them.