Fix "'int' object is not iterable" error in set_device_id_for_pushers background update (#16594)

A regression from removing the cursor_to_dict call, adds back
the wrapping into a tuple.

Author: clokep

changelog.d/16594.bugfix

@@ -0,0 +1 @@
+Fix "'int' object is not iterable" error in `set_device_id_for_pushers` background update introduced in Synapse 1.95.0.

synapse/storage/databases/main/pusher.py

@@ -601,7 +601,7 @@ def set_device_id_for_pushers_txn(txn: LoggingTransaction) -> int:
                 (last_pusher_id, batch_size),
             )
 
-            rows = txn.fetchall()
+            rows = cast(List[Tuple[int, Optional[str], Optional[str]]], txn.fetchall())
             if len(rows) == 0:
                 return 0
 
@@ -617,7 +617,7 @@ def set_device_id_for_pushers_txn(txn: LoggingTransaction) -> int:
                 txn=txn,
                 table="pushers",
                 key_names=("id",),
-                key_values=[row[0] for row in rows],
+                key_values=[(row[0],) for row in rows],
                 value_names=("device_id", "access_token"),
                 # If there was already a device_id on the pusher, we only want to clear
                 # the access_token column, so we keep the existing device_id. Otherwise,