TestBaseCasting.test_to_numpy fails for ExtensionArrays that yield nested numpy arrays
The test case fails when data is e.g. `np.array([np.array([np.nan, None], dtype="object"), None], dtype="object")`. 

```python
def test_to_numpy(self, data):
        expected = np.asarray(data)

        result = data.to_numpy()
>       self.assert_equal(result, expected)

```

This happens in `fletcher` for the `list<item: string>` type.