setup: Re-create venv with Python 3.10 when upgrading to stbt_version 33
setup: Re-create venv with Python 3.10 when upgrading to stbt_version 33

If the venv's Python version isn't 3.10, *and* Python 3.10 is actually
available, we delete the ".venv" directory and re-create it with Python
3.10.

stb-tester/stbt-rig