Fix 100

[ewohnlich]

Fix case where index value is changed to None after previously being indexed. Values of type None are still not allowed, but it should now also unindex a previous value if one existed.

[d-maurer]

Definitely an improvement.

In your test, maybe, you could use self.assertRaises (as context manager -- i.e. with self.assertReaises(KeyError)) instead of the tedious try .. except else.