Breaks pypi2nix

[lovesegfault]

Issue description

I'm adding this issue a few days after it happened, so it's no longer fresh on my head but basically there is some weird interaction between this and pypi2nix where it (pypi2nix) fails to launch the shell with the generated Python environment

Steps to reproduce

1. Pick a PyPI project (e.g. https://github.com/beetbox/beets)
2. pypi2nix (You may need -r requirements.txt depending on the project)

Technical details

Please run the following commands and paste the results:

1. command nix-shell -p nix-info --run "nix-info -m"

 - system: `"x86_64-linux"`
 - host os: `Linux 5.4.5, NixOS, 20.03pre206137.f5bb6c02071 (Markhor)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.3.1`
 - channels(root): `"nixos-20.03pre206137.f5bb6c02071"`
 - channels(bemeurer): `"home-manager"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

2. zsh --version

zsh 5.7.1 (x86_64-pc-linux-gnu)

[chisui]

Sorry, but I'm not a python guy. Can you provide more detailed steps to reproduce your issue? The beets project doesn't contain a requriements.txt and in the travis config they seem to manually install their dependencies.