env/deps: python tools in "externally managed environments"

[matzf]

On recent Debian/Ubuntu systems (and possibly others), installing anything with pip install, even with --user, as we do in tools/env/pip3/deps, results in an error looking roughly like this:

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.

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.

More details:
https://peps.python.org/pep-0668/
https://packaging.python.org/en/latest/specifications/externally-managed-environments/

---

As a simple workaround we can add --break-system-packages to our installation script tools/env/pip3/deps. I'm fairly confident that installing the few packages that we need with pip install --user does not actually break system packages, but the flag is intentionally named so as to "carry some connotation that its use is risky". Setting this flag does not seem like a good permanent solution.

Possible alternatives for the dependencies installed in tools/env/pip3:

• supervisor and supervisor-wildcards; for this, the distribution packages in recent Debian/Ubuntu/Fedora/EPEL seem sufficiently up to date (https://packages.debian.org/search?keywords=supervisor, https://packages.ubuntu.com/search?keywords=supervisor, https://packages.fedoraproject.org/pkgs/supervisor/supervisor/), but not available on Amazon Linux 2023 (currently used in buildkite CI) -- if necessary, we can do something customized for the installation on the CI.
• toml, pyyaml and plumbum, as dependencies of the topogen scripts. To avoid the installation in the user site packages, the topogen scripts could be run in a venv, or (*shudder*) via with bazel and python_rules.