title: Set user's secret key in Django Rest Framework JWT
body: <p>I am using <a href="https://jpadilla.github.io/django-rest-framework-jwt/" rel="nofollow noreferrer">Django REST framework JWT
  </a> for authentication in my Django Rest API.</p>
  
  <p>I want to use user-password as the user's secret key so that tokens become unusable when the user changes the password. According to <a href="https://jpadilla.github.io/django-rest-framework-jwt/" rel="nofollow noreferrer">this documentation</a>, I have to set the value of JWT_GET_USER_SECRET_KEY for it and value should be a function, accepting user as the only parameter and returning its secret key. The documentation doesn't have an example of such a function.</p>
  
  <p>Should I just write a function that takes a user instance and return the password?
  Can someone give an example of how this function should be written?</p>