Issue description

let
  py = python3.override {
    packageOverrides = self: super: {
      affinity = super.affinity.overridePythonAttrs (oldAttrs: rec {
        disabled = false;
        doCheck = false;
      });

This fails to evaluate as disabled = isPy3k; is set in the definition for affinity.

Cc @FRidh - any thoughts?

Technical details

• system: "x86_64-linux"
• host os: Linux 4.18.12, NixOS, 19.03.git.3a22e3007f6M (Koi)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.1.3