_ClientList: add a __setitem__
_ClientList: add a __setitem__

Also from pytype:

File "/home/tycho/packages/qtile/libqtile/layout/tile.py", line 95, in shift: unsupported operand type(s) for item assignment: 'libqtile.layout.base._ClientList' [unsupported-operands]
  No attribute '__setitem__' on libqtile.layout.base._ClientList

It seems reasonable to have this since it's a thin wrapper around a list,
so let's add it.

Signed-off-by: Tycho Andersen <6b523322678240882acbb4da5312a12d8cd0082d@tycho.ws>

zordsdavini/qtile,zordsdavini/qtile