Update flake8 config
Update flake8 config

add rule F401 (unused imports) to ignored rules for config modules

volunteer-planner/volunteer_planner,volunteer-planner/volunteer_planner,christophmeissner/volunteer_planner,coders4help/volunteer_planner,christophmeissner/volunteer_planner,pitpalme/volunteer_planner,volunteer-planner/volunteer_planner,christophmeissner/volunteer_planner,pitpalme/volunteer_planner,pitpalme/volunteer_planner,coders4help/volunteer_planner,coders4help/volunteer_planner,christophmeissner/volunteer_planner,pitpalme/volunteer_planner,volunteer-planner/volunteer_planner,coders4help/volunteer_planner