add pip executable suffix on windows
add pip executable suffix on windows

Fixes #303
Connected to #303 and #297

paolodedios/pybuilder,arcivanov/pybuilder,pybuilder/pybuilder,arcivanov/pybuilder,esc/pybuilder,esc/pybuilder,esc/pybuilder,paolodedios/pybuilder,pybuilder/pybuilder,pybuilder/pybuilder,arcivanov/pybuilder,paolodedios/pybuilder