Attaching a patch that fixes the slice deletion problems.

The approach I've taken is follow a similar implementation in Objects/listobject.c; when a slice is deleted, it can be done efficiently by using memmove to shift whole blocks of leftover children.

Also added a new test class to test_xml_etree specifically to test slicing with Element objects and deleting slices with strange steps.

I'll leave it up for review for a couple of days before committing.