py3: Add Python 2/3 compat module
py3: Add Python 2/3 compat module

Keep all the hacks in a single place. This looks like all we need, so no need
to depend on six.

priestd09/mopidy,jcass77/mopidy,hkariti/mopidy,rawdlite/mopidy,rawdlite/mopidy,mokieyue/mopidy,bacontext/mopidy,bencevans/mopidy,bacontext/mopidy,quartz55/mopidy,glogiotatidis/mopidy,rawdlite/mopidy,swak/mopidy,ali/mopidy,jodal/mopidy,mokieyue/mopidy,dbrgn/mopidy,tkem/mopidy,diandiankan/mopidy,SuperStarPL/mopidy,jcass77/mopidy,mopidy/mopidy,bacontext/mopidy,tkem/mopidy,priestd09/mopidy,pacificIT/mopidy,bencevans/mopidy,dbrgn/mopidy,vrs01/mopidy,swak/mopidy,adamcik/mopidy,bencevans/mopidy,SuperStarPL/mopidy,glogiotatidis/mopidy,jodal/mopidy,ZenithDK/mopidy,priestd09/mopidy,ali/mopidy,vrs01/mopidy,ali/mopidy,quartz55/mopidy,adamcik/mopidy,pacificIT/mopidy,pacificIT/mopidy,ali/mopidy,jcass77/mopidy,diandiankan/mopidy,jmarsik/mopidy,tkem/mopidy,ZenithDK/mopidy,quartz55/mopidy,kingosticks/mopidy,SuperStarPL/mopidy,mopidy/mopidy,adamcik/mopidy,mopidy/mopidy,hkariti/mopidy,kingosticks/mopidy,bencevans/mopidy,swak/mopidy,pacificIT/mopidy,glogiotatidis/mopidy,jmarsik/mopidy,jmarsik/mopidy,quartz55/mopidy,vrs01/mopidy,tkem/mopidy,vrs01/mopidy,mokieyue/mopidy,bacontext/mopidy,dbrgn/mopidy,diandiankan/mopidy,SuperStarPL/mopidy,jodal/mopidy,hkariti/mopidy,jmarsik/mopidy,glogiotatidis/mopidy,rawdlite/mopidy,ZenithDK/mopidy,hkariti/mopidy,swak/mopidy,ZenithDK/mopidy,dbrgn/mopidy,kingosticks/mopidy,mokieyue/mopidy,diandiankan/mopidy