Include `poetry.lock` in git tracking for reproducible builds

[mitchmindtree]

I've been hacking on a Nix flake for dasy, and early on while attempting to use poetry2nix, having the poetry.lock committed was necessary for poetry2nix to generate a nix derivation.

I ended up taking a different route so no longer need this, but thought I'd open this anyways as I imagine other folks using poetry might find it handy to make sure they can reproduce the same build? That said I'm not too familiar with poetry, feel free to close if unnecessary/uninterested!