Around the beginning of 2025, the `ubuntu-latest` runner image which [this project uses][1], [was changed from Ubuntu 22.04 to 24.04][2].

As a result, the python version which is used by this repo's GitHub actions changed from [3.10.12][3] to [3.12.3][4].

With this change to Python 3.12, [PEP 668][5] was introduced. This new Python behavior is causing errors in the CI and `pip` is exiting non-zero

```
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/python3.13/README.venv for more information.
```

This commit changes the CI to use virtualenvs to address this externally-managed-environment problem.

This also fixes up or updates CI by

* Changing the build image from Ubuntu 20.04 to 24.04
* Updating the list of Python versions tested to just the current supported Python versions
* Updating the versions of the `checkout` and `setup-python` actions
* Anchoring the install image to Ubuntu 24.04 (instead of `latest`) to prevent this type of thing in the future
* Updating container OSes for install
  * CentOS Stream 8 to CentOS Stream 9 as Stream 8 is end of lifed
  * Ubuntu focatl to noble

[1]: https://github.com/systemd/python-systemd/blob/903142423452c4dd18110b7f8a953dabb2031e49/.github/workflows/install.yml#L17
[2]: actions/runner-images#10636
[3]: https://github.com/actions/runner-images/blob/main/images/ubuntu/Ubuntu2204-Readme.md#installed-software
[4]: https://github.com/actions/runner-images/blob/main/images/ubuntu/Ubuntu2404-Readme.md#language-and-runtime
[5]: https://peps.python.org/pep-0668/