turn deprecated slice methods into wrappers
turn deprecated slice methods into wrappers

__delslice__() and __setslice__() are deprecated as of python v2.6 [1]
and urwid does not support python versions pre-v2.6 (anymore).

Making these methods wrappers of __delitem__() and __setitem__()
de-duplicates code (and doctest's)

[1] https://docs.python.org/2.6/reference/datamodel.html

wardi/urwid,urwid/urwid,foreni-packages/urwid,tonycpsu/urwid,hkoof/urwid,foreni-packages/urwid,douglas-larocca/urwid,hkoof/urwid,inducer/urwid,rndusr/urwid,douglas-larocca/urwid,drestebon/urwid,hkoof/urwid,rndusr/urwid,inducer/urwid,douglas-larocca/urwid,foreni-packages/urwid,urwid/urwid,drestebon/urwid,inducer/urwid,wardi/urwid,urwid/urwid,tonycpsu/urwid,drestebon/urwid,rndusr/urwid,wardi/urwid,tonycpsu/urwid