Fix tox mypy check

Author: MadLittleMods

synapse/events/builder.py

@@ -100,7 +100,7 @@ def is_state(self):
     async def build(
         self,
         prev_event_ids: List[str],
-        overriding_prev_events: False,
+        overriding_prev_events: Literal[False] = False,
         auth_event_ids: Optional[List[str]],
     ) -> EventBase:
         """Transform into a fully signed and hashed event