Add TypeVars for the Iterators in select.select

[marsfan]

Back in 2017, #1080 added in TypeVar to handle correctly indicating how the arguments for select.select were passed through to the return values. But unfortunately it caused issues for when an empty Iterator was provided for an argument, as mypy could not resolve the type, so it was reverted in #1097

But now, TypeVar has an optional default argument that is used to set a default when the type is not provided, and we have the Never type to indicate a case where there is an empty list. By combining the two , it is now possible to support more descriptive types.

[github-actions]

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

[srittau]

Thanks! It might be safer (for gradual typing) to use a default of Any, but since there are no primer hits, we can try this variant first, and still "downgrade" to Any if it causes problems.

[marsfan]

@srittau Sounds like a plan.

I think (but I may be wrong) that if we useAny is that it does not make it as clear that if you pass in an empty array for an argument, then the output array will also be empty, so hopefully Never does not cause any issues.