change deprecated slice methods into wrappers

__delslice__() and __setslice__() are deprecated as of python v2.6 [1]. Making
them wrappers of __delitem__ and __setitem__(), de-duplicates code.

[1] https://docs.python.org/2.6/reference/datamodel.html
Add doctest examples of negative indices for __delitem__()
__delitem__ support negative single index