What would the advantage of that be in an isolated environment like this?

Compatability with the framework being developed in https://github.com/scientific-python/cookie for testing.

Can you explain that a bit more? It feels unhelpful to layer more venv tools ontop of an already isolated env, but I don't know much about this

At Astropy we've stopped supporting our own package template in favor of the scientific-python package guide. They have developed https://github.com/scientific-python/cookie. It would be great to increase compatibility with their work, since we officially endorse it. I would be happy however that happens, so I opened this PR to start requesting support for some differences I noticed between our testing frameworks (another is nox versus tox).

https://github.com/scientific-python/cookie/blob/main/{{cookiecutter.project_name}}/.github/workflows/ci.yml

So do you mean we would run tox/nox using pipx rather than pip install tox and run it separately?