2020 Pi 3 Mount extended

Just a quick remix to extend the mounting holes out past the raspberry pi so it doesn't have to be removed to get to the mounting screws. Also corrected the holes for the nuts as they were too small. And I have added a remixed version of adamwdraper's top case https://www.thingiverse.com/thing:2539269 to finish it off. This uses 4 2.5mm x 16mm screws and nuts.