If _typeshed/foo.pyi defines _Bar, then can we do from _typeshed.foo import _Bar? This might be a more standard way to mark stable/unstable things than a # stable comment.

_typeshed is still mostly for internal use. I'd also expect types to become stable after they've proven themselves in _typeshed and I don't want to do a big search/replace through all of typeshed when we mark a type as stable. This would also make _typeshed more unstable, because it changes more often.

The CI failure is unrelated and likely caused by python/mypy#10507.

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