Invalid implementation of `del my_list[a:b]`

Deleting slices does not behave like Python's lists:

```python
>>> standard_list = [1,2,3]
>>> del standard_list[:2]
>>> standard_list
[3]
>>> sparse_list = SparseList([1,2,3])
>>> del sparse_list[:2]
>>> sparse_list
[None, None, 3]
```