Can't install pyside for python3

[igsha]

Issue description

The dependent package pyside-apiextractor-0.10.10 for pyside uses libxml2 which is not supported for python3.

Steps to reproduce

Just install python3Package.pyside:

[18:36] igor@nixos-pc .nixpkgs (master *) $ nix-shell -p python3Packages.pyside --show-trace
error: while evaluating the attribute ‘buildInputs’ of the derivation ‘shell’ at /nix/store/dvxlg0zbpvkn8qv9dpqvia56cbvknipj-nixos-17.09pre106299.7f3b857d0d/nixos/pkgs/build-support/trivial-builders.nix:7:14:
while evaluating the attribute ‘buildInputs’ of the derivation ‘python3.5-pyside-1.2.4’ at /nix/store/dvxlg0zbpvkn8qv9dpqvia56cbvknipj-nixos-17.09pre106299.7f3b857d0d/nixos/pkgs/development/interpreters/python/mk-python-derivation.nix:56:3:
while evaluating the attribute ‘buildInputs’ of the derivation ‘pyside-generatorrunner-0.6.16’ at /nix/store/dvxlg0zbpvkn8qv9dpqvia56cbvknipj-nixos-17.09pre106299.7f3b857d0d/nixos/pkgs/development/python-modules/pyside/generatorrunner.nix:4:3:
while evaluating the attribute ‘buildInputs’ of the derivation ‘pyside-apiextractor-0.10.10’ at /nix/store/dvxlg0zbpvkn8qv9dpqvia56cbvknipj-nixos-17.09pre106299.7f3b857d0d/nixos/pkgs/development/python-modules/pyside/apiextractor.nix:4:3:
while evaluating ‘getOutput’ at /nix/store/dvxlg0zbpvkn8qv9dpqvia56cbvknipj-nixos-17.09pre106299.7f3b857d0d/nixos/lib/attrsets.nix:450:23, called from undefined position:
while evaluating ‘getCrossDrv’ at /nix/store/dvxlg0zbpvkn8qv9dpqvia56cbvknipj-nixos-17.09pre106299.7f3b857d0d/nixos/pkgs/stdenv/generic/default.nix:133:21, called from undefined position:
libxml2 not supported for interpreter python3.5m

Technical details

• System: NixOS: 17.09pre106299.7f3b857d0d (Hummingbird)
• Nix version: nix-env (Nix) 1.11.9
• Nixpkgs version: 17.09pre106299.7f3b857d0d

[FRidh]

if I am correct this package just needs a working Python env to run some things. It doesn't necessarily need to be the Python from the specific package set.