@rgommers taught me about a new pip install flag we'll want to use with py-scipy. Here is my understanding of the history of this flag (@pradyunsg please correct me if I'm wrong, I'm probably oversimplifying things):

1. Once upon a time, pip install was simply a wrapper around python setup.py install. If you wanted to pass extra options to the build, you could use pip install --global-option=<global> --install-option=<install>, which would in turn run python setup.py <global> install <install>.
2. PEP 517 added a way to specify a "build backend". Instead of running python setup.py install, if a build-backend is specified in pyproject.toml, it is invoked directly. However, there was no way to pass options to a build backend.
3. Add a UI to set config settings for PEP 517 backends pypa/pip#11059 added a --config-settings flag to pip (first available in 22.1 release) which allows you to pass extra options to the build backend.

There is one important thing to note about this flag. It was first added in pip 22.1, which only supports Python 3.7+. This means that any package we add config_settings to will only support Python 3.7+. For py-scipy, we will only add this flag to scipy 1.9+, which already only supports Python 3.7+, so there's no issue there. But for other packages which specify a build-backend in pyproject.toml, this raises the question of which Python versions we are willing to support. Opened #31824 to discuss this without getting side-tracked with this PR.