Fixup syntax for Python 3.7

Author: David Robertson

tests/http/federation/test_srv_resolver.py

@@ -49,7 +49,8 @@ def do_lookup() -> Generator["Deferred[object]", object, List[Server]]:
 
             with LoggingContext("one") as ctx:
                 resolve_d = resolver.resolve_service(service_name)
-                result: List[Server] = yield defer.ensureDeferred(resolve_d)  # type: ignore[assignment]
+                result: List[Server]
+                result = yield defer.ensureDeferred(resolve_d)  # type: ignore[assignment]
 
                 # should have restored our context
                 self.assertIs(current_context(), ctx)