fix: poetry wouldn't work when using a tag

[teto]

fixes #160

I copy/pasted the patch from my fork of nixpkgs here and could not properly test.

[teto]

@adisbladis I am trying to write tests for this to convince you (and also fixed an issue in my previous PR).
I've been running nix-build tests/default.nix but if fails with unknown package cppy ?
upon entering nix-shell, nixpkgs is overriden to one that doesn't seem valid but if I restore nixpkgs to my local checkout with export NIX_PATH=nixpkgs=/home/teto/nixpkgs (a nixos-unstable fork) tests work again. Not sure what's wrong, I never used niv.

[teto]

while writing the test #171 for this, it seemed like poetry works already fine so don't merge this yet. I wonder if the error I had was not a sideeffect of python-poetry/poetry#2062 in fact.

[teto]

doesn't seem needed after all, I wonder if I got mystified by a mismatched locked file and pyproject.toml. Closing.