use stringed template vars on PY3 in convert plugin
use stringed template vars on PY3 in convert plugin

We decode them with `surrogateescape`, so we avoid decoding errors

SusannaMaria/beets,jackwilsdon/beets,shamangeorge/beets,diego-plan9/beets,beetbox/beets,jackwilsdon/beets,lengtche/beets,SusannaMaria/beets,SusannaMaria/beets,sampsyo/beets,jackwilsdon/beets,beetbox/beets,beetbox/beets,pkess/beets,madmouser1/beets,beetbox/beets,jackwilsdon/beets,artemutin/beets,xsteadfastx/beets,lengtche/beets,lengtche/beets,diego-plan9/beets,diego-plan9/beets,madmouser1/beets,ibmibmibm/beets,Kraymer/beets,pkess/beets,mosesfistos1/beetbox,mosesfistos1/beetbox,shamangeorge/beets,ibmibmibm/beets,sampsyo/beets,MyTunesFreeMusic/privacy-policy,diego-plan9/beets,MyTunesFreeMusic/privacy-policy,Kraymer/beets,ibmibmibm/beets,xsteadfastx/beets,xsteadfastx/beets,pkess/beets,Kraymer/beets,mosesfistos1/beetbox,mosesfistos1/beetbox,shamangeorge/beets,artemutin/beets,artemutin/beets,madmouser1/beets,madmouser1/beets,MyTunesFreeMusic/privacy-policy,SusannaMaria/beets,sampsyo/beets,sampsyo/beets,lengtche/beets,shamangeorge/beets,Kraymer/beets,artemutin/beets,MyTunesFreeMusic/privacy-policy,xsteadfastx/beets,ibmibmibm/beets,pkess/beets