Fix failing test

Author: Victorien PLOT

tests/middleware/test_middleware.py

@@ -1,8 +1,10 @@
 from starlette.middleware import Middleware
+from starlette.types import Receive, Scope, Send
 
 
 class CustomMiddleware:
-    pass
+    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
+        return None
 
 
 def test_middleware_repr():