This problem seems to be related to the test suite of Elpy upstream, 
which has been failing for months, consecutively, since 2024 (see 
https://github.com/jorgenschaefer/elpy/actions?query=workflow%3ATests). 
Apparently, the project is unmaintained (see 
https://github.com/jorgenschaefer/elpy):

 > IMPORTANT NOTE: Looking for a maintainer
 >
 > I find myself (@galaunay, current maintainer), unable to maintain 
Elpy at the moment, and probably for a while. If you are interested in 
getting involved in Elpy, please contact me by mail, the project 
definitely needs you !
 >
 > In the meantime, I will keep an eye on the PRs to integrate any fix 
and/or new features proposed, but I will definitely not be able to treat 
the issues in a satisfying manner.

The last successful emacs-elpy build for x86_64-linux systems in Guix is 
from September 3, 2024 (see 
https://ci.guix.gnu.org/search?query=spec%3Amaster+system%3Ax86_64-linux+emacs-elpy). 
After that, all builds seem to be failing for the same reason: «command 
"ert-runner" failed with status 255».

Maybe emacs-elpy should be removed from Guix?


§ WORKAROUNDS

I wasn't affected until recently because I was using an older revision 
of Guix where emacs-elpy worked normally. But today I installed 
emacs-elpy as follows

   guix package -i emacs-elpy --without-tests=emacs-elpy

So far, it seems to be working fine. If it turns out to fail at some 
point, I think I'll use the guix time-machine.