python3Packages.pynacl: doesn't cross-compile

[legendofmiracles]

Describe the bug

The package python3Packages.pynacl can't be cross compiled to other architectures, tested: raspberry-pi and aarch-multiplatform

Steps To Reproduce

Steps to reproduce the behavior:

1. nix build nixpkgs#pkgsCross.raspberryPi.python3Packages.pynacl

Expected behavior

It should be compiled.

Screenshots

$ nix build nixpkgs#pkgsCross.raspberryPi.python3Packages.pynacl
error: builder for '/nix/store/vs3s6kmri734axcxcf9pk90ccd1vrpb8-python3.9-pynacl-1.4.0-armv6l-unknown-linux-gnueabihf.drv' failed with exit code 1;
       last 10 log lines:
       >     add_cffi_module(dist, cffi_module)
       >   File "/nix/store/bvwj3jd0ddnjm0lv12smkb16dz4c5rvc-python3.9-cffi-1.14.5-armv6l-unknown-linux-gnueabihf/lib/python3.9/site-packages/cffi/setuptools_ext.py", line 49, in add_cffi_module
       >     execfile(build_file_name, mod_vars)
       >   File "/nix/store/bvwj3jd0ddnjm0lv12smkb16dz4c5rvc-python3.9-cffi-1.14.5-armv6l-unknown-linux-gnueabihf/lib/python3.9/site-packages/cffi/setuptools_ext.py", line 25, in execfile
       >     exec(code, glob, glob)
       >   File "src/bindings/build.py", line 36, in <module>
       >     ffi = FFI()
       >   File "/nix/store/bvwj3jd0ddnjm0lv12smkb16dz4c5rvc-python3.9-cffi-1.14.5-armv6l-unknown-linux-gnueabihf/lib/python3.9/site-packages/cffi/api.py", line 48, in __init__
       >     import _cffi_backend as backend
       > ModuleNotFoundError: No module named '_cffi_backend'
       For full logs, run 'nix log /nix/store/vs3s6kmri734axcxcf9pk90ccd1vrpb8-python3.9-pynacl-1.4.0-armv6l-unknown-linux-gnueabihf.drv'.

Additional context

This doesn't seem to a nix-only problem, but still influenced by nix.

Doesn't compile on arm: pyca/pynacl#654
AIX: pyca/pynacl#555
Some missing support for aarch64: pyca/pynacl#601
Doesn't compile via qemu for arm32v7 and arm64v8: pyca/pynacl#553

The error I'm having doesn't seem to show up anywhere else though. And a few of those threads also post work arounds.

Notify maintainers

There is no maintainer.

System Information

$ nix-shell -p nix-info --run "nix-info -m"
this path will be fetched (0.05 MiB download, 0.28 MiB unpacked):
/nix/store/xq77wmzk8d306anyk9rsvl4i4h7ihchw-bash-interactive-4.4-p23-dev
copying path '/nix/store/xq77wmzk8d306anyk9rsvl4i4h7ihchw-bash-interactive-4.4-p23-dev' from 'https://cache.nixos.org'...

• system: "x86_64-linux"
• host os: Linux 5.13.12-xanmod1-cacule, NixOS, 21.11 (Porcupine)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.4pre20210802_47e96bb
• channels(root): ""
• channels(nix): ""
• nixpkgs: /nix/store/02cm621rs5wbnmvbq79b12f08hs8xrf0-xfw6vsapwykrrcqhjs4zi4kzwhgji4hx-source

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute: python3Packages.pynacl
# a list of nixos modules affected by the problem
module:

[legendofmiracles]

It seems to be that every package that declares cffi in it's propagatedBuildInputs is failing to cross compile. If I put it in nativeBuildInputs it fails with another error though:

     > Executing pipInstallPhase
      > /build/PyNaCl-1.4.0/dist /build/PyNaCl-1.4.0
      > Processing ./PyNaCl-1.4.0-cp39-cp39-linux_armv6l.whl
      > Requirement already satisfied: cffi>=1.4.1 in /nix/store/lf0ydk1cjzgrsll3j3vx3yngmvmwcnrd-python3.9-cffi-1.14.5/lib/python3.9/site-packages (from PyNaCl==1.4.0) (1.14.5)
      > ERROR: Could not find a version that satisfies the requirement six (from pynacl) (from versions: none)
      > ERROR: No matching distribution found for six

Should I open another, conglomerate, issue for all the other packages?