flit-core as new dependency upstream

flit-core as new dependency upstream. Waiting for feedback: https://gitlab.mister-muffin.de/josch/img2pdf/issues/212#issuecomment-2780

_Originally posted by @dreua in https://github.com/flathub/com.github.jeromerobert.pdfarranger/issues/330#issuecomment-3497082164_


Does anyone here have an idea if we should rather build this or just patch the requirements to remove it?