Add some type hints to event_federation datastore (#12753)

Co-authored-by: David Robertson <david.m.robertson1@gmail.com>

changelog.d/12753.misc

@@ -0,0 +1 @@
+Add some type hints to datastore.

mypy.ini

@@ -27,7 +27,6 @@ exclude = (?x)
    |synapse/storage/databases/__init__.py
    |synapse/storage/databases/main/cache.py
    |synapse/storage/databases/main/devices.py
-   |synapse/storage/databases/main/event_federation.py
    |synapse/storage/schema/
 
    |tests/api/test_auth.py

synapse/handlers/room_batch.py

@@ -53,6 +53,7 @@ async def inherit_depth_from_prev_ids(self, prev_event_ids: List[str]) -> int:
         # We want to use the successor event depth so they appear after `prev_event` because
         # it has a larger `depth` but before the successor event because the `stream_ordering`
         # is negative before the successor event.
+        assert most_recent_prev_event_id is not None
         successor_event_ids = await self.store.get_successor_events(
             most_recent_prev_event_id
         )
@@ -139,6 +140,7 @@ async def get_most_recent_full_state_ids_from_event_id_list(
             _,
         ) = await self.store.get_max_depth_of(event_ids)
         # mapping from (type, state_key) -> state_event_id
+        assert most_recent_event_id is not None
         prev_state_map = await self.state_store.get_state_ids_for_event(
             most_recent_event_id
         )