title: Airflow HdfsSensor operator is not working
body: <p>I'm trying to use Hdfs sensor operator in Airflow to trigger the next tasks based on the file arrival in the given path. But, when I deploy the dag, getting the error as </p>
  
  <p><strong>Broken DAG: [/usr/local/airflow/dags/test_sensor_dag.py] invalid syntax (client.py, line 1473)</strong>
  Code: </p>
  
  <pre><code>from airflow import DAG
  from airflow.sensors.hdfs_sensor import HdfsSensor
  from datetime import datetime
  
  
  default_args = {'owner': 'airflow',
                  'depends_on_past': False,
                  'provide_context': True,
                  'start_date': datetime(2020, 3, 19, 0, 0),
                  'email': ['hr@***.com'],
                  'email_on_failure': True,
                  'email_on_retry': False,
                  'retries': 0,
                  'concurrency': 1
                  }
  
  # run it daily at 6AM
  schedule_interval = '00 6 * * *'
  
  dag_name = 'test_sensor_dag'
  
  dag = DAG(
      dag_id=dag_name,
      default_args=default_args,
      schedule_interval=schedule_interval)
  
  source_data_sensor = HdfsSensor(
      task_id='source_data_sensor',
      filepath='/data/test/file.csv',
      poke_interval=10,
      timeout=5,
      dag=dag
  ).poke()
  
  success_notification = EmailOperator(to=['hr@***.com'], task_id='success_notification',
                                       subject='[Success:] test for {{ ds }}',
                                       html_content='Successfully ran the DAG',
                                       dag=dag)
  
  source_data_sensor
  success_notification.set_upstream(source_data_sensor)
  </code></pre>