kombu.exceptions.EncodeError: <User: testuser> is not JSON serializable

CELERY_BROKER_URL = 'amqp://localhost'
CELERY_RESULT_BACKEND = 'amqp://localhost'
CELERY_ACCEPT_CONTENT = ['application/json']
CELERY_RESULT_SERIALIZER = 'json'
CELERY_TASK_SERIALIZER = 'json'
CELERY_TIMEZONE = 'Asia/Makassar'
CELERY_BEAT_SCHEDULE = {}

from __future__ import absolute_import, unicode_literals
from celery import task
from django.contrib.auth.models import User


@task(serializer='json')
def task_number_one():
    user = User.objects.create(username="testuser", email="test@test.com", password="pass")
    return user

def form_valid(self, form):
    form.instance.user = self.request.user
    task_number_one.delay()
    return super().form_valid(form)