Check the secret key checksum in PGPPacket._parse_generic_secret_key_packet

Check the secret key checksum in PGPPacket._parse_generic_secret_key_packet

From RFC 4880 [1]:

  - If the string-to-key usage octet is zero or 255, then a two-octet
    checksum of the plaintext of the algorithm-specific portion (sum
    of all octets, mod 65536).  If the string-to-key usage octet was
    254, then a 20-octet SHA-1 hash of the plaintext of the
    algorithm-specific portion.  This checksum or hash is encrypted
    together with the algorithm-specific fields (if string-to-key
    usage octet is not zero).  Note that for all other values, a
    two-octet checksum is required.

[1]: section 5.5.3