But the test suite actually picks the 'python-packaging' that is
propagated from 'pytest', and promptly fails.

Updating packaging alone is also a staging-level change, so I did not
dig much further.  But there is some room for improvement here.  Maybe
by (setenv "PYTHONPATH" (string-append ".:" (getenv "PYTHONPATH"))) ?

I also wonder whether pytest needs to propagate packaging at all, or if
wrapping is enough.  Do you know how common it is to "import pytest" or
"from pytest import foo", and whether those code paths would need
'packaging'?

Anyway, future problems!  Thanks for maintaining Poetry.  :-)