If we don't find another Python version to use, our last-resort fallback should be the latest supported, not earliest supported, Python version. This minimizes first-experience false positives; if users need to support older Python versions, they can configure that to opt in to stricter checking.

(We can also of course add new version sources that take priority over this default, such as looking at the venv Python version; that's independent of this issue.)

Holding off on this change while we figure out Ruff's python-version handling, in case it influences this choice in some way, but filing this issue into the Alpha milestone so we don't miss it before the alpha.