bump sync ver to 6 (see note)
bump sync ver to 6 (see note)

when porting this to other platforms, please ensure you have ported all
changes in sched.py after 2.0.12 first

hssm/anki,sunclx/anki,LucasCabello/anki,holycrepe/anki,abeyer/anki,weihautin/anki,jakesyl/ruby-card,ospalh/libanki3,jkitching/anki,go38/anki,jakesyl/ruby-card,Arthaey/anki,weihautin/anki,kenjhim/anki,eduOS/anki,socialpercon/anki-1,Stvad/anki,xuewenfei/anki,subfusc/anki,hssm/anki,florianjacob/anki