test updates
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several things:
 - now explicitly decoding to latin-1 when trying to decode strings
 - Issuer payload is now forced to 8 octets in length
 - moved the cool docstring for Header.parse that I made a long time ago 'cause I like it, into the new Header.parse
 - commented out ElGSignature for the time being
 - finished SignatureV4(VersionedPacket)
 - removed some commented lines that no longer needed to be kept around
 - added some reserved constants to HashAlgorithm
 - added Unknown to KeyServerPreferences; made it an IntEnum