fix bad practice mutable kwarg default
fix bad practice mutable kwarg default

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