feat: Support Nix

[l0b0]

To test on for example Ubuntu:

1. Install Nix
2. cd into the project directory
3. Run nix-shell

This enters a new nested shell with pre-commit and all the Python packages installed.

[imincik]

@l0b0 , I am still not sure if poetry2nix in particular is ready to be adopted by all of us. There is quite steep learning curve for Nix in general and I think poetry2nix needs more time to mature.

[l0b0]

@l0b0 , I am still not sure if poetry2nix in particular is ready to be adopted by all of us. There is quite steep learning curve for Nix in general and I think poetry2nix needs more time to mature.

Can you point to any specific issues with poetry2nix? It builds, and the resulting Nix shell works. I don't think repeating all the packages in a Nix expression is better than poetry2nix.

Edit: Actually, there is still one issue when running coverage. Let's just freeze this change for now - I can always get back to it later.