lnutil.UpdateAddHtlc: use attrs instead of old-style namedtuple

electrum/util.py

@@ -277,6 +277,9 @@ class MyEncoder(json.JSONEncoder):
     def default(self, obj):
         # note: this does not get called for namedtuples :(  https://bugs.python.org/issue30343
         from .transaction import Transaction, TxOutput
+        from .lnutil import UpdateAddHtlc
+        if isinstance(obj, UpdateAddHtlc):
+            return obj.to_tuple()
         if isinstance(obj, Transaction):
             return obj.serialize()
         if isinstance(obj, TxOutput):