– Install virtualenv on Python 3.9

Add dead-snakes repository:

```shell-script
sudo add-apt-repository ppa:deadsnakes/ppa
```

Install Python 3.9, development package and the specific venv:

```shell-script
sudo apt install python3.9
sudo apt install python3.9-dev
sudo apt install python3.9-venv
```

Create Python 3.9 virtualenv:

```shell-script
python3.9 -m venv py39
```

Install everything from the requirements.txt file (created via pip freeze > requirements.txt)

pip install -r requirements.txt