Cannot read a Transferable Secret Key with unknown secret key encryption mechanism

[twiss]

Reading the following key:

-----BEGIN PGP PRIVATE KEY BLOCK-----
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=3w/O
-----END PGP PRIVATE KEY BLOCK-----

results in the error openpgp: invalid data: first packet was not a public/private key.

In some cases, it can be useful to be able to read such a private key, in order to derive a Transferable Public Key, for example.

See ProtonMail/gosop#28.

[dkg]

gentle nudge, this is almost a year old, but it seems like it might be a simple change, and fixing it (being able to handle an unknown S2K usage octet) should reduce brittleness.