title: pymongo : can't use resume token stored in Redis
body: <p>I am trying to use pymongo to listen to a change stream while being resilient to failures / restarts. Thus, I am storing the resume token in Redis for later reuse.</p>
  <p>Here is the code I tried</p>
  <pre><code># Skipping pymongo and redis initialisation
  
  if redis.exists('resumeToken'):
     resume_token = pickle.loads(redis.get('resumeToken'))
  else:
     resume_token = None
  stream = db.my_collection.watch(
              pipeline=[{'$match': {'fullDocument.type': 'something'}}],
              resume_after=resume_token,
              full_document='updateLookup',
              max_await_time_ms=500
          )
  change = stream.next()
  redis.set('resumeToken', pickle.dumps(change['_id']))
  print(change)
  </code></pre>
  <p>The first time it runs, a change is printed and I get something stored in Redis.</p>
  <p>But if I re-run, I got the following error:</p>
  <p><code>pymongo.errors.OperationFailure: The resume token UUID does not exist. Has the collection been dropped?</code></p>
  <p>I am using MongoDB 3.6.1 ad pymongo 3.7.1-1.1.</p>
  <p>Anyone has an idea why my resume token is rejected ?</p>