The problem is that in all the cases of the normal `dict` or `OrderedDict` the returned object has a `set` interface, so this code should work for any `dict`:

```
from six import viewkeys
viewkeys(d) - {'a', 'b'}
```

The above code fails for Python3 with:

```
TypeError: unsupported operand type(s) for -: 'list' and 'set'
```

When using the `six.viewkeys()` it maps it to `viewkeys() for Python2 and `keys()` for Python3.

In Python2 `dict().viewkeys()` returns `dict_keys([])`.
In Python3 `dict().keys()` also returns `dict_keys([])`.

For Python2: `OrderedDict().viewkeys()` returns `KeysView(OrderedDict())`
For Python3: ``OrderedDict().keys()` returns `odict_keys([])`

The `SON` class returns `list` for `SON().keys()`.