Expose template as flake attribute

[mayl]

This allows users to setup your template with:

nix flake init --template github:oceansprint/pydev

[mayl]

Leaving this as draft for now - it seems like your template cannot create a nix-shell in an empty project. I'm not enough of a python dev to know the right steps, but if you put a pyproject.toml or whatever else is needed to make a full blank project under the template directory this would work with all those files.