The version of MarkupSafe must be chosen depending on the version of
Python that runs the 'pip' subprocess, and not the current 'buildout'
process.

This makes a difference when e.g. buildout runs with Python 2, because
[python3] defaults to Python 3.8+ and we want to install the recent
MarkupSafe version (even if Python 2 can only install the old one).