Add six version requirement
Add six version requirement

six.add_metaclass isn't available in all versions of six.

rxcomm/qtile,flacjacket/qtile,ramnes/qtile,ramnes/qtile,kseistrup/qtile,dequis/qtile,frostidaho/qtile,w1ndy/qtile,soulchainer/qtile,frostidaho/qtile,tych0/qtile,aniruddhkanojia/qtile,kiniou/qtile,apinsard/qtile,farebord/qtile,StephenBarnes/qtile,nxnfufunezn/qtile,rxcomm/qtile,kiniou/qtile,himaaaatti/qtile,de-vri-es/qtile,w1ndy/qtile,aniruddhkanojia/qtile,andrewyoung1991/qtile,farebord/qtile,tych0/qtile,zordsdavini/qtile,himaaaatti/qtile,encukou/qtile,jdowner/qtile,apinsard/qtile,kynikos/qtile,qtile/qtile,de-vri-es/qtile,xplv/qtile,soulchainer/qtile,EndPointCorp/qtile,cortesi/qtile,dequis/qtile,flacjacket/qtile,cortesi/qtile,qtile/qtile,xplv/qtile,EndPointCorp/qtile,andrewyoung1991/qtile,kopchik/qtile,nxnfufunezn/qtile,jdowner/qtile,StephenBarnes/qtile,zordsdavini/qtile,encukou/qtile,kopchik/qtile,kynikos/qtile,kseistrup/qtile