chore: replace pip3 install with pipx install

[SethFalco]

Using pip3 install to install PyPI packages can cause headache, or worse, break system packages on Linux distributions.

Vladimyr did a good write-up about it on our Matrix chat:
https://matrix.to/#/!HiSKnvVsCpWxkTWWMC:gitter.im/$_8RsjYQBQIIcOGfhrJNm2fvex3O9tWnn0lhIb8riX5M?via=gitter.im&via=matrix.org&via=one.ems.host

Debian would also yell at me if I tried to follow those instructions:

pip3 install tldr

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.

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.

Meanwhile, I'd recommend suggesting pipx instead of pip3.

https://github.com/pypa/pipx

This creates a venv behind the scenes, isolating the package and it's dependencies from other system-wide packages, so there's no risk of conflict. In Python-land, it's also a common way to install binaries anyway, for example Poetry (a very popular package/project manager for Python) also recommends installing it with pipx.