[kernFeatureWriter] add default __future__imports; fix sorted .items() and `filter` having no len()
[kernFeatureWriter] add default __future__imports; fix sorted .items() and `filter` having no len()

jamesgk/ufo2fdk,googlefonts/ufo2ft,jamesgk/ufo2ft,moyogo/ufo2ft,googlei18n/ufo2ft