bip327: minor fixes

- An error test vector doesn’t specify the InvalidContributionError type
- In *DeterministicSign*, use GetXonlyPubkey instead of GetPubkey
- The key_agg_and_tweak fn doesn’t specify the return type
- In partial_sig_verify_internal, the pubkey arg should be PlainPk
- Remove unused enumerate() fn calls
- In test_sign_verify, add an additional assert statement

bip-0327/vectors/sig_agg_vectors.json

@@ -143,7 +143,8 @@
             ],
             "error": {
                 "type": "invalid_contribution",
-                "signer": 1
+                "signer": 1,
+                "contrib": "psig"
             },
             "comment": "Partial signature is invalid because it exceeds group size"
         }