Install virtualenv on Python 3.9
Add dead-snakes repository:
sudo add-apt-repository ppa:deadsnakes/ppa
Install Python 3.9, development package and the specific venv:
sudo apt install python3.9
sudo apt install python3.9-dev
sudo apt install python3.9-venv
Create Python 3.9 virtualenv:
python3.9 -m venv py39