Remove explicit target-version field for ruff

[srittau]

This is inferred from project.requires-python

[srittau]

Part of #15358.

[AlexWaygood]

This is the same as project.requires-python now, but it seems semi-plausible that we might again have a situation in the future where the version we require for our tests (project.requires-python) is higher than our minimum supported Python version for our stubs — and we probably want the target version for ruff to reflect the latter rather than the former? Explicitly specifying the target version here for Ruff might be helpful for reminding us that these two things don't necessarily need to be the same.

That said, I don't have a strong opinion; either is probably fine

[srittau]

This is the same as project.requires-python now, but it seems semi-plausible that we might again have a situation in the future where the version we require for our tests (project.requires-python) is higher than our minimum supported Python version for our stubs — and we probably want the target version for ruff to reflect the latter rather than the former?

My main problem with target-version is that it's easy to overlook, since its format (310) differs from the usual format (3.10). I also wonder whether we should really use project.requires-python for the tests, and the custom field tool.typeshed.oldest_supported_python for the project itself. This seems to be more of an historical accident than a logical use of the fields.

For now, lets remove it. It can easily be restored later.