Reference cycles in TraitListObject

After code resembling this:

```python
class Model(HasStrictTraits):
foo = List()

model = Model()
my_list = model.foo
```

`my_list` is now an object of type `TraitListObject`, and is involved in the following reference cycle:


We should see if we can find a way to eliminate the reference cycle. (If it's easy to do so, we should definitely do it. If it turns out to be hard to do reliably, then we'll have to think about what the best path forward is.)