bug#60733: python build systems does not support cross-compilation

[Maxim Cournoyer]

Hello,

Here's an example:

--8<---------------cut here---------------start------------->8---
$ guix build --target=aarch64-linux-gnu python-coverage

guix build: error: gnu/packages/check.scm:1997:2: python-coverage@5.2.1:
build system `python' does not support cross builds
--8<---------------cut here---------------end--------------->8---

Python byte compiled files are not architecture dependents, so nothing
is needed for them.  In some cases where C extensions are used,
setuptools or PEP 517 python build systems handle this transparently.

So it seems like we should be able to support cross-compiling Python
packages easily, which can be useful when building images containing
Python packages for embedded systems via cross-compilation.

Thoughts?

-- 
Thanks,
Maxim