$ pypi2nix -v -v -V 3.5 -r requirements.txt
pypi2nix v1.8.0 running ...

Stage1: Downloading wheels and creating wheelhouse ...
|-> nix-shell /nix/store/05csqr2d2sn6i01v0s7j8s2s20ajg4kk-pypi2nix-1.8.0/pkgs/pypi2nix/pip.nix --arg requirements_files [ "/run/user/1000/pypi2nix/985076f2ab55a3d874fc963f526cb436/b989feef0fbc12ef9f54853b27134f51.txt" ] --argstr wheel_cache_dir /run/user/1000/pypi2nix/cache/wheel --argstr pip_build_dir /run/user/1000/pypi2nix/cache/pip --arg extra_build_inputs [  ] --argstr python_version python35 --argstr project_dir /run/user/1000/pypi2nix/985076f2ab55a3d874fc963f526cb436 --argstr extra_env  --argstr download_cache_dir /run/user/1000/pypi2nix/cache/download --arg setup_requires [  ] -I nixpkgs=/home/ryan/.nixpkgs/nixpkgs:nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs:nixos-config=/etc/nixos/configuration.nix:/nix/var/nix/profiles/per-user/root/channels --show-trace --pure --run exit
    warning: Nix search path entry ‘/home/ryan/.nixpkgs/nixpkgs:nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs:nixos-config=/etc/nixos/configuration.nix:/nix/var/nix/profiles/per-user/root/channels’ does not exist, ignoring
    Ignoring importlib: markers 'python_version <= "2.7"' don't match your environment
    Ignoring ordereddict: markers 'python_version < "2.7"' don't match your environment
    Collecting Babel==2.3.4 (from -r /run/user/1000/pypi2nix/985076f2ab55a3d874fc963f526cb436/b989feef0fbc12ef9f54853b27134f51.txt (line 4))
      Using cached Babel-2.3.4.tar.gz
    Collecting beautifulsoup4==4.4.1 (from -r /run/user/1000/pypi2nix/985076f2ab55a3d874fc963f526cb436/b989feef0fbc12ef9f54853b27134f51.txt (line 5))
      Using cached beautifulsoup4-4.4.1.tar.gz
    Collecting blinker==1.3 (from -r /run/user/1000/pypi2nix/985076f2ab55a3d874fc963f526cb436/b989feef0fbc12ef9f54853b27134f51.txt (line 6))
      Using cached blinker-1.3.tar.gz
    Collecting click==6.6 (from -r /run/user/1000/pypi2nix/985076f2ab55a3d874fc963f526cb436/b989feef0fbc12ef9f54853b27134f51.txt (line 7))
      Using cached click-6.6.tar.gz
    Collecting extras==0.0.3 (from -r /run/user/1000/pypi2nix/985076f2ab55a3d874fc963f526cb436/b989feef0fbc12ef9f54853b27134f51.txt (line 8))
      Using cached extras-0.0.3.tar.gz
    Collecting fixtures==2.0.0 (from -r /run/user/1000/pypi2nix/985076f2ab55a3d874fc963f526cb436/b989feef0fbc12ef9f54853b27134f51.txt (line 9))
      Using cached fixtures-2.0.0.tar.gz