Building Newelle without Flatpak
Newelle can be built without Flatpak, however there might be some bugs. The list of requirements (both python and non python) is present here modules/requirements.txt. Newelle manages its own python path for some python modules, this means that dependencies marked as optional can still be installed inside Newelle, or managed by it. This is useful to know if your target distribution does not packages those dependencies.
How to build
meson setup builddir --prefix=/usr/bin
ninja -C builddir
sudo ninja -C builddir install
Then you can start it with
./builddir/src/newelle