Document policy for future Python versions

[srittau]

Closes: #12569

[AlexWaygood]

We could also consider adding an AST-based lint to check_typeshed_structure.py enforcing this (but that doesn't have to be done in this PR). I don't think such a lint would be a good fit for flake8-pyi since it would be enforcing a typeshed-specific policy

[JelleZijlstra]

We could also consider adding an AST-based lint to check_typeshed_structure.py enforcing this (but that doesn't have to be done in this PR). I don't think such a lint would be a good fit for flake8-pyi since it would be enforcing a typeshed-specific policy

I'd prefer to not add any such lint checks directly to typeshed, and leave those to tools that are good at linting. In flake8-pyi we could add a configuration option like allowed_versions = ["3.8", "3.14"] and make it warn about version checks outside that range, so other projects could set their own policy.