make the exec plugin fully py3 compatible
make the exec plugin fully py3 compatible

resolves #99

esc/pybuilder,arcivanov/pybuilder,elkingtonmcb/pybuilder,alex-dow/pybuilder,Danielweber7624/pybuilder,pybuilder/pybuilder,elkingtonmcb/pybuilder,Designist/pybuilder,arcivanov/pybuilder,onesfreedom/pybuilder,TimYi/pybuilder,Danielweber7624/pybuilder,locolupo/pybuilder,arcivanov/pybuilder,locolupo/pybuilder,Designist/pybuilder,onesfreedom/pybuilder,esc/pybuilder,TimYi/pybuilder,alex-dow/pybuilder,pybuilder/pybuilder,paolodedios/pybuilder,paolodedios/pybuilder,pybuilder/pybuilder,esc/pybuilder,paolodedios/pybuilder