Add scrypt KDF

[geitda]

Add a new KDF algorithm type for scrypt. Field names are pulled exactly from RFC7914.
Added some logic (but no direct tests for it) in EncryptionAlgorithm.kdf_hmac and EncryptionAlgorithm.kdf_iterations to return None as scrypt has no concept of those, but as it has both a salt and (optional) key_length parameter, EncryptionAlgorithm.kdf_salt and EncryptionAlgorithm.key_length can be left as-is. I created a test case modeled after the ccm_parameters test - it loads an entire EncryptionAlgorithm but only tests the KdfAlgorithm part we're interested in. Again, additional tests for the kdf_* and key_length properties may be prudent. Creating other files to test with is easy using openssl pkcs8 -scrypt and any of the optional switches that go with that (-scrypt_N N, -scrypt_r r, or -scrypt_p p)

[wbond]

Sorry that I missed this earlier. Could you rebase on master so we can try and get all of the CI jobs to run? For some reason only a couple did.

[wbond]

Awesome, thank you for this enhancement!