Consider adding a "TopLevel" keyword that stands in for the top level bound of a corresponding type variable

Summary

Please consider a new typing keyword that stands in for the top level type of a corresponding type variable.

Motivation

Consider a test here and the corresponding definition of DistributionInfo here. The test loses all typing information because I used DistributionInfo[Any, Any, Any]. So if I want typing information, I have to use:

def test_flatten(generator: Generator,
                 distribution_info: DistributionInfo[NaturalParametrization[ExpectationParmetrization, JaxComplexArray],
                                                     ExpectationParametrization[NaturalParmetrization],
                                                     NumpyComplexArray]) -> None:

I would much rather just do

def test_flatten(generator: Generator,
                 distribution_info: DistributionInfo[TopLevel, TopLevel, TopLevel]) -> None:

It's not just shorter, but it reduces churn if the type definitions change. Note that TopLevel is recursive (it chooses the top level of the nested types too).

Start with the linked tests/test_flatten.py and tests/distribution_info.py to understand the typing information lost when DistributionInfo[Any, Any, Any] is used. Clarify the intended recursive semantics of TopLevel and how it differs from existing type-variable behavior. Done means the proposal has a precise specification and an agreed validation path.