title: Python package APSscheduler throws an error when starting scheduler if mongodb is used as a jobstore
body: <p>I'm trying to run some code with APScheduler with MongoDB as a jobstore. I've downloaded pymongo and I tested it, so it does work. APScheduler will work when I instantiate the scheduler, but when I run scheduler.start() it throws the following error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "aps_ro.py", line 56, in &lt;module&gt;
      scheduler.start()
    File "/usr/lib/python3.8/site-packages/apscheduler/schedulers/background.py", line 33, in start
      BaseScheduler.start(self, *args, **kwargs)
    File "/usr/lib/python3.8/site-packages/apscheduler/schedulers/base.py", line 158, in start
      store.start(self, alias)
    File "/usr/lib/python3.8/site-packages/apscheduler/jobstores/mongodb.py", line 57, in start
      self.collection.ensure_index('next_run_time', sparse=True)
    File "/usr/lib/python3.8/site-packages/pymongo/collection.py", line 2028, in ensure_index
      self.__create_index(keys, kwargs, session=None)
    File "/usr/lib/python3.8/site-packages/pymongo/collection.py", line 1881, in __create_index
      with self._socket_for_writes(session) as sock_info:
    File "/usr/lib/python3.8/site-packages/pymongo/collection.py", line 195, in _socket_for_writes
      return self.__database.client._socket_for_writes(session)
    File "/usr/lib/python3.8/site-packages/pymongo/mongo_client.py", line 1266, in _socket_for_writes
      server = self._select_server(writable_server_selector, session)
    File "/usr/lib/python3.8/site-packages/pymongo/mongo_client.py", line 1253, in _select_server
      server = topology.select_server(server_selector)
    File "/usr/lib/python3.8/site-packages/pymongo/topology.py", line 233, in select_server
      return random.choice(self.select_servers(selector,
    File "/usr/lib/python3.8/site-packages/pymongo/topology.py", line 192, in select_servers
      server_descriptions = self._select_servers_loop(
    File "/usr/lib/python3.8/site-packages/pymongo/topology.py", line 208, in _select_servers_loop
      raise ServerSelectionTimeoutError(
  pymongo.errors.ServerSelectionTimeoutError: localhost:27017: [Errno 111] Connection refused
  </code></pre>
  
  <p>My actual code sample is as follows:</p>
  
  <pre><code>jobstores = {
      'mongo': {'type': 'mongodb'},
      'default': SQLAlchemyJobStore(url='sqlite:///jobs.sqlite')
  }
  executors = {...}
  job_defaults = {...}
  
  scheduler = BackgroundScheduler()
  scheduler.configure(jobstores=jobstores, executors=executors, job_defaults=job_defaults, timezone=utc)
  
  @scheduler.scheduled_job(trigger='cron', day_of_week=4, hour=22, minute=21, second=0)
  def tester():
      print("hello")
  
  scheduler.start()
  </code></pre>