[SOLVED kind of ] Python3.9 Big Sur Unified installer problem

I have upgraded a Mac to Big Sur and have installed (with brew) Python3.9.

If I try to install Plone 5.2 (universal installer), I end up with error:

Detailed installation log
Starting at Sat Dec 18 15:32:33 CET 2021
Creating Python virtual environment.
Error: Command '['/Applications/Plone/zinstance/bin/python3.9', '-Im', 'ensurepip', '--upgrade', '--default-pip']' returned non-zero exit status 1.
Virtualenv successfully created
Installing Python requirements in virtual environment.
sudo: /Applications/Plone/zinstance/bin/pip: command not found
Unable to install Python requirements in virtual environment.


Any suggestions?

UPDATE: It looks like it is possible to ./install.sh standalone (not as sudo) and after the buildout fails: cd /to/folder/of Plone; python -m venv . ; pin install the requirements and run buildout. That said: Plone < 5.2.4 installs, while 5.2.6 errors with 'not enough values to unpack'.

Now I need to go and buy things the family can unpack and hopefully 'value'…

UPDATE: Change of requirements ( setuptools etc) fixes the 'not enough values to unpack' error