TestBaseCasting.test_to_numpy fails for ExtensionArrays that yield nested numpy arrays

[xhochy]

The test case fails when data is e.g. np.array([np.array([np.nan, None], dtype="object"), None], dtype="object").

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.