bip327: minor fixes

I made some minor corrections, which I believe are beneficial. Please let me know if any of them are incorrect or need modification.

Changes:

1. An error test vector doesn’t specify the InvalidContributionError type
2. In DeterministicSign, use GetXonlyPubkey instead of GetPubkey (doesn’t exist)
3. The key_agg_and_tweak fn doesn’t specify the return type
4. In partial_sig_verify_internal, the pubkey arg type should be PlainPk rather than bytes
5. Remove unnecessary enumerate() calls
   • I suspect this was initially used in debugging to find the failing test vector, and then it was forgotten.
6. In test_sign_verify, add an additional assert statement that checks if two pubnonces add up to the infinity aggnonce.

cc @jonasnick @real-or-random