Update Python installation instructions in community-training.md

[MakisH]

There are several more places on the website and our documentation in general that need updating (including the Python bindings), this is only focusing on the training page.

Note: I am wondering if the suggestion to create the venv as .venv conflicts with the run scripts that also call python -m venv .venv (and later try to re-source the venv). I can eventually try this.

[MakisH]

I tried following the instructions for the CHT part of the course (Nutils), and having a venv already activated does not conflict: the run.sh script of solid-nutils creates and activates a new venv in a local directory .venv and re-installs the requirements, using cached packages.

Creating the venv directly in the place where the run script automatically creates a new venv does not matter as well (even if this is not a realistic use case), as the directory is not overwritten, and no error is thrown.

If anything, we might need to improve the tutorials assumptions.