fluxbox is missing in Debian/testing (upcoming bookworm) due to a ftbfs
bug (see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=984134) since
2021-10-27, and it looks like this isn't going to change soonish. We've
good experience with usage of openbox for custom Grml flavours, so let's
try replacing fluxbox with openbox (and obconf for configuring openbox).