[cmake][pyscript] Ignore warnings when importing packages for version check

Importing virtualenv (20.2.2) triggers

/usr/local/lib/python3.9/site-packages/setuptools/command/install.py:34: SetuptoolsDeprecationWarning: setup.py install is deprecated. Use build and pip and other standards-based tools. warnings.warn(

for me. After some search, I found this is triggered in cmake/scripts/pyversion.py during dunecontrol. Probably a bug on their side printing such a warning no import but it's a strange warning to get when configuring Dune so I'd rather disable any such warnings in the future. The script just want to get the version after all.