gh-91162: Disallow tuple[T][*anything] by mrahtz · Pull Request #92255 · python/cpython

We agreed in #91162 that unpacked type arguments should only be valid to generic aliases that can accept a variable number of arguments - that is, unpacked type arguments should only be valid to generic aliases with a TypeVarTuple in the type parameters.

@JelleZijlstra Could I ask you for review on this?

(This PR is based on #92249, so there are a couple of extra commits. See the latest commit for the one that's actually part of this PR. I think I should be able to effectively hide the changes from the extra commits once #92249 is merged - so we can start review on this PR now?)