Implementation of websocket API for importer

I'm playing around with websockets with the ultimate goal of building a web importer frontend for the beets importer process. Currently I'm trying to get a proof of concept to work using Flask-SocketIO with eventlet. For this it's necessary to monkey patch several stdlib modules to 'green' them so they're compatible with eventlet.

However, when I perform the monkey patch I get an error:
```
queues = [CountedQueue(queue_size) for i in range(queue_count)]
File "/opt/cucumba/venv/lib/python3.6/site-packages/beets/util/pipeline.py", line 95, in __init__
queue.Queue.__init__(self, maxsize)
File "/opt/cucumba/venv/lib/python3.6/site-packages/eventlet/green/Queue.py", line 15, in __init__
super(Queue, self).__init__(maxsize)
TypeError: super(type, obj): obj must be an instance or subtype of type
```
After some analysis and trial and error, I got things to work by altering the init method of `CountedQueue` in pipeline.py:

```diff
95c95
< queue.Queue.__init__(self, maxsize)
---
> super(CountedQueue, self).__init__(maxsize)
```

That's nice, but I have a few questions still:
1. Is there any good reason why the `super()` call was not used in the first place, as seems general practice in the file (and community?)
2. I honestly still don't precisely understand the exact differences in the original call and my replaced `super()` call. Could someone elaborate on that?
3. I don't understand why the original situation leads to the specific error.
4. Is this adjustment okay for the pipeline (somewhere in the future)?

Trying to learn and contribute :).

Thanks,
Bart