Add a regression test for TypeVars that should be _typeshed.Self

[AlexWaygood]

My attempt at a regression test for #6880, presented for your consideration. I'm aware that this would be a significant addition to check_new_syntax, so I understand if you don't want to merge this!

I've deliberately reintroduced some bad syntax, so you can see what it looks like when the test fails.

[JelleZijlstra]

Thanks, I think checks like these are helpful.

I wish we'd put them in flake8-pyi though, so they're more reusable and we can use standard linter infrastructure (like noqa instead of hardcoding specific paths).

[AlexWaygood]

Thanks, I think checks like these are helpful.

I wish we'd put them in flake8-pyi though, so they're more reusable and we can use standard linter infrastructure (like noqa instead of hardcoding specific paths).

I'm happy to file a PR over at flake8-pyi! This felt like a typeshed-specific change though (_typeshed is a private module, right?)

[JelleZijlstra]

_typeshed can also be used in third-party stub packages, and error codes should make it easy for third-party users to control whether to enable this check.

[AlexWaygood]

Great, I'll close this and refile over at flake8-pyi then :)

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉