use new backendspawner

custom/4.0.0/requirements.txt

 jsonformatter==0.3.1
-jupyterhub-backendspawner==0.3.3
+jupyterhub-backendspawner==0.4.0
 oauthenticator==14.2.0
 # nodeenv==1.6.0

custom/4.0.0/spawner/api_setup_tunnel.py

@@ -63,7 +63,7 @@ class SetupTunnelAPIHandler(APIHandler):
             )
 
             json_body["servername"] = spawner.name
-            json_body["svc_port"] = await spawner.get_port()
+            json_body["svc_port"] = spawner.port
             json_body["svc_name"] = spawner.get_service_address().split(".")[0]
             labels = {
                 "hub.jupyter.org/username": user.name,