Error in Airflow DAG

mydag.py

from __future__ import print_function
from builtins import range
from airflow.operators import PythonOperator, PostgresOperator, DummyOperator
from airflow.models import DAG
from datetime import datetime, timedelta

import time
from pprint import pprint

seven_days_ago = datetime.combine(
        datetime.today() - timedelta(7), datetime.min.time())

args = {
    'owner': 'airflow',
    'start_date': seven_days_ago,
}

dag = DAG(
    dag_id='anders_test_dag', default_args=args,
    schedule_interval=None)

def hello_world(ds, **kwargs):
    print(ds)
    return 'Hello World!'

task1 = PythonOperator(
    task_id='hello_world',
    provide_context=True,
    python_callable=hello_world,
    dag=dag)

task2 = PostgresOperator(
    task_id='postgrestest',
    provide_context=True,
    autocommit=True,
    sql="DELETE * from myschema.mytable",
    postgres_conn_id="my-datastore",
    dag=dag
)

task2.set_upstream(task1)