Add remaining type hints to synapse.events. (#11098)

Author: clokep

changelog.d/11098.misc

@@ -0,0 +1 @@
+Add type hints to `synapse.events`.

mypy.ini

@@ -22,13 +22,7 @@ files =
   synapse/config,
   synapse/crypto,
   synapse/event_auth.py,
-  synapse/events/builder.py,
-  synapse/events/presence_router.py,
-  synapse/events/snapshot.py,
-  synapse/events/spamcheck.py,
-  synapse/events/third_party_rules.py,
-  synapse/events/utils.py,
-  synapse/events/validator.py,
+  synapse/events,
   synapse/federation,
   synapse/groups,
   synapse/handlers,