title: Find sublists of innermost lists in 3d list
body: <p>I have a list <code>l=[[[1,2,3],[4,5,6]],[[7,8,9],[9,1,2]]]</code>
  How do I get the list <code>[[[1,2],[4,5]],[[7,8],[9,1]]]</code> using just slicing? I tried <code>l[:][:][:2]</code> but it gave me the entire 3d list.</p>