Install python3 and python3-venv if missing

Setting up axii on a new Ubunutu VM results in

tobi@ubuntu2:~/axii$ ./bootstrap.sh
/usr/bin/python3
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Verifying apt dependencies ...
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dpkg: package 'python3-venv' is not installed
Please install python3 and python3-venv:
sudo apt install python3 python3-venv

axii could perform sudo apt install python3 python3-venv automatically after asking the user for confirmation.