fix mypy

fastapi/routing.py

@@ -120,10 +120,10 @@ def _merge_lifespan_context(
     async def merged_lifespan(app: AppType) -> AsyncIterator[Mapping[str, Any]]:
         async with original_context(app) as maybe_self_context:
             async with nested_context(app) as maybe_nested_context:
-                context = maybe_self_context or {}
-                if maybe_nested_context:
-                    context.update(maybe_nested_context)
-                yield context
+                yield {
+                    **(maybe_self_context or {}),
+                    **(maybe_nested_context or {})
+                }
 
     return merged_lifespan