As far as I can tell, there has never been a need or request for UserList slice support in its 17 year history.  And since the collections ABCs were added, this class has mostly fallen into disuse and probably doesn't warrant API expansion (the module only narrowly missed being omitted from Python 3 all together).

Aside from the question of whether this should be done (I am -0 on it), this patch looks correct as far as it goes.  If slice support does get added to __getitem__, it should also be conferred on __setitem__ and __delitem__ as well.

Besides the question of whether anyone would ever need this, there is also a minor performance consideration in that it will mildly degrade the performance of any existing code that uses UserList.