stack: fix attribute error
stack: fix attribute error

Caught by pytype:

File "/home/tycho/packages/qtile/libqtile/layout/stack.py", line 38, in __str__: No attribute 'current' on _WinStack [attribute-error]

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

ramnes/qtile,qtile/qtile,qtile/qtile,tych0/qtile,ramnes/qtile,tych0/qtile