Rabbitmq MQTT Bad username or bad password - rabbitmq

Question

I installed Rabbit MQ on my Ubuntu 16.04 Server. After that, I enabled plugins for MQTT for RabbitMQ. In rabbitmq-plugins list I can see that MQTT plugin is enabled and running on the server.
I then added the following configuration file for MQTT at this location
/etc/rabbitmq/rabbitmq.config
and restarted the server:
[{rabbit, [{tcp_listeners, [5672]}]},
{rabbitmq_mqtt, [{default_user, <<"myuser">>},
{default_pass, <<"mypass">>},
{allow_anonymous, false},
{vhost, <<"/">>},
{exchange, <<"amq.topic">>},
{subscription_ttl, 1800000},
{prefetch, 10},
{ssl_listeners, []},
%% Default MQTT with TLS port is 8883
%% {ssl_listeners, [8883]}
{tcp_listeners, [1883]},
{tcp_listen_options, [{backlog, 128},
{nodelay, true}]}]}
].
Now When I am trying to publish a message to rabbitmq server like:
import paho.mqtt.publish as publish
import paho.mqtt.client as mqtt
publish.single('/',
payload='hello world',
hostname='xxx.xxx.xxx.xxx', # My servers IP address
auth={'username':'myuser', 'password':'mypass'},
port=1883,
protocol=mqtt.MQTTv311)
It gives me this error:
paho.mqtt.MQTTException: Connection Refused: bad user name or password.
There is no Encryption for now. So what am I doing wrong?
I tried the same procedure with Mosquitto MQTT Broker and it worked fine. I think the issue is with my rabbitmq configuration.