tox plugin broken on noble

[Jürgen Gmach]

```
Running ['bash', '--noprofile', '--norc', '-ec', 'python3 -m pip install tox==3.24.5; tox']
:: error: externally-managed-environment
::
:: × This environment is externally managed
:: ╰─> To install Python packages system-wide, try apt install
:: python3-xyz, where xyz is the package you are trying to
:: install.
::
:: If you wish to install a non-Debian-packaged Python package,
:: create a virtual environment using python3 -m venv path/to/venv.
:: Then use path/to/venv/bin/python and path/to/venv/bin/pip. Make
:: sure you have python3-full installed.
::
:: If you wish to install a non-Debian packaged Python application,
:: it may be easiest to use pipx install xyz, which will manage a
:: virtual environment for you. Make sure you have pipx installed.
::
:: See /usr/share/doc/python3.12/README.venv for more information.
::
:: note: If you believe this is a mistake, please contact your Python installation or OS distribution provider. You can override this, at the risk of breaking your Python installation or OS, by passing --break-system-packages.
:: hint: See PEP 668 for the detailed specification.
```

Ubuntu (and Debian) introduced PEP 668, which prevents installing packages in the global Python interpreter/site packages. For more context see https://pythonspeed.com/articles/externally-managed-environment-pep-668/.

There are several possible solutions:
- manually create a virtual env and use that to install tox into (disadvantage: cumbersome)
- use tox via system packages (disadvantage: different versions for different Ubuntu series)
- install pipx via system packages and then install tox via pipx (preferred way)