Walking <class 'aaf2.components.NestedScope'> not implemented

### Incorrect Functionality and General Questions

I try to read structure of the aaf file timeline, and I got the next error.
```
/usr/local/lib/python3.7/dist-packages/aaf2/components.py in walk(self)
217 else:
218 raise NotImplementedError("Walking {} not implemented".format(
--> 219 type(segment)))
220
221 @register_class

NotImplementedError: Walking not implemented
```

## To Reproduce

1. Google Colab
2. Python 3.7
3. 0.13.0

```
timeline = otio.adapters.read_from_file("Test_zapis_34_5.aaf")
```

## Additional Context

My aaf file https://drive.google.com/file/d/1sj66C3u_uj_5zmbBsEijqKB4zh98gyQd/view?usp=sharing. I think this problem occured when I generate aaf file with nested sequence. Aaf file generated with Premiere Pro 2019 (13.1.4)