keyset not byte array

cashu/core/crypto.py

@@ -30,6 +30,6 @@ def derive_pubkeys(keys: Dict[int, PrivateKey]):
 def derive_keyset_id(keys: Dict[str, PublicKey]):
     """Deterministic derivation keyset_id from set of public keys."""
     pubkeys_concat = "".join([p.serialize().hex() for _, p in keys.items()])
-    return base64.b64encode(hashlib.sha256((pubkeys_concat).encode("utf-8")).digest())[
-        :12
-    ]
+    return base64.b64encode(
+        hashlib.sha256((pubkeys_concat).encode("utf-8")).digest()
+    ).decode()[:12]