Fix for Python >=3.6
Fix for Python >=3.6

Python 3.6 drops the deprecated `inspect.getargspec()` function.  For
Python >= 3.3, use the prefered `inspect.signature()`.

The nightly Travis bulid is still failing, but this is a problem with
Nose.

himaaaatti/qtile,rxcomm/qtile,dequis/qtile,dequis/qtile,frostidaho/qtile,qtile/qtile,soulchainer/qtile,frostidaho/qtile,kynikos/qtile,jdowner/qtile,cortesi/qtile,tych0/qtile,ramnes/qtile,kynikos/qtile,rxcomm/qtile,de-vri-es/qtile,zordsdavini/qtile,cortesi/qtile,qtile/qtile,zordsdavini/qtile,himaaaatti/qtile,flacjacket/qtile,jdowner/qtile,ramnes/qtile,de-vri-es/qtile,flacjacket/qtile,soulchainer/qtile,tych0/qtile