Niv appears to work too, but I haven't tested that as much.

Changes (sorry single big commit, but this was hard enough as is):
- don't use the overlay for nixpkgs since it's not used ATM
- when using a flake, respect the given inputs instead of using Niv ones
- update dependencies to Qt6 to match `pyproject.toml`
- fix coredump on Wayland because of missing runtime dependency
- fix build-system to use the newer `poetry-core`
- update poetry.lock
- make flake inputs and Niv sources poetry2nix versions match

Example:
"QApplication: invalid style override 'kvantum' passed, ignoring it."

`prev.callPackage` auto-detects the given closure's args and sets them
to `prev.${arg}`. So `poetry2nix` was getting overriden with
`prev.poetry2nix` giving us the "poetry2nix is now maintained out-of-tree"
error.

Python dependency update is required to build with latest nixpkgs.

Co-authored-by: adminy <[email protected]>

And make Nix package use the lxml from nixpkgs, to allow building with
a more recent version.