Check that the test suite is writing requires-python with the correct version

[nathanjmcdougall]

Motivation
I noticed that when a test creates a pyproject.toml file, it might create it with a lower bound that differs from the CI matrix Python version. I think ideally we would force it to match, at least in most cases.

Summary of desired enhancement
Check out whether this is happening, get a failing test, and then fix it.