Fix use of pip in testing workflow

[speth]

Currently, to run the test suite after installing the packages, we run:

pip3 install pytest pint

I think we are using pip because the versions in Ubuntu 20.04 are too old.

For Ubuntu 23.04, this no longer works, with pip giving the error:

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/python3.11/README.venv for more information.

Trying to create a venv further insists on the package python3-venv. This seems to also work in 20.04.