title: 'CeleryExecutor' is not defined even after pip install airflow[celery]
body: <p>I am trying to switch over from LocalExecutor to CeleryExecutor on my local machine. I should have the necessary libraries installed using  <code>pip install airflow[celery]</code>. I also have redis installed and running.</p>
  
  <p>However, when I try to run <code>airflow worker</code>, I get the error:</p>
  
  <pre><code>DEFAULT_EXECUTOR = CeleryExecutor()
  NameError:  CeleryExecutor' is not defined
  </code></pre>
  
  <p>I have a <code>broker url</code> and <code>celery_result_backend</code> both set to <code>redis://localhost:6379</code> with redis running in the background. What am I doing wrong?</p>