Add tests for reading ZIP method 20 (PKWare zstd)

[emmatyping]

The zipfile code should be plumbed through to actual read this, perhaps we can generate an archive and store it in test data, since writing is not possible.

[emmatyping]

So as far as I can tell, nothing actually supports method 20, not even PKZip. So in 9129c5c I removed all references to it.