Virtual environment (venv)

Also written: venv, .venv, virtual environment, activate, deactivate

A private folder of Python and packages for one project, so projects can't break each other. Create it with `python3 -m venv .venv`, then activate it in each new terminal with `source .venv/bin/activate` (macOS/Linux) or `.venv\Scripts\activate` (Windows); `deactivate` turns it off.