bug#70281: Incorrect python-portend output version

[Sergio Pastor Pérez]

Hello.

I've noticed that `python-portend' does not have a correct version
number in the output derivation. This will create failures during the
`python-build-system' `sanity-check' phase.

The following command demonstrates the issue:
--8<---------------cut here---------------start------------->8---
$ guix shell --pure grep python python-portend -- pip3 list portend | grep 
'portend'
portend          0.0.0
...
--8<---------------cut here---------------end--------------->8---

Have a great day,
Sergio.