Convert Transaction and Edu object to attrs (#10542)

Instead of wrapping the JSON into an object, this creates concrete
instances for Transaction and Edu. This allows for improved type
hints and simplified code.

synapse/federation/sender/transaction_manager.py

@@ -27,6 +27,7 @@ from synapse.logging.opentracing import (
     tags,
     whitelisted_homeserver,
 )
+from synapse.types import JsonDict
 from synapse.util import json_decoder
 from synapse.util.metrics import measure_func
 
@@ -104,13 +105,13 @@ class TransactionManager:
                 len(edus),
             )
 
-            transaction = Transaction.create_new(
+            transaction = Transaction(
                 origin_server_ts=int(self.clock.time_msec()),
                 transaction_id=txn_id,
                 origin=self._server_name,
                 destination=destination,
-                pdus=pdus,
-                edus=edus,
+                pdus=[p.get_pdu_json() for p in pdus],
+                edus=[edu.get_dict() for edu in edus],
             )
 
             self._next_txn_id += 1
@@ -131,7 +132,7 @@ class TransactionManager:
             # FIXME (richardv): I also believe it no longer works. We (now?) store
             #  "age_ts" in "unsigned" rather than at the top level. See
             #  https://github.com/matrix-org/synapse/issues/8429.
-            def json_data_cb():
+            def json_data_cb() -> JsonDict:
                 data = transaction.get_dict()
                 now = int(self.clock.time_msec())
                 if "pdus" in data: