gh-91162: Implement splitting of *tuple[int, ...] over T, *Ts in typing.py

[mrahtz]

In #91162, we decided that it should be valid to do

T=TypeVar('T') Ts=TypeVarTuple('Ts') Alias=tuple[T, *Ts] Alias[*tuple[int, ...]]

and that the result should be tuple[int, *tuple[int, ...]].

This PR does the first half of the work necessary to enable this: the changes necessary in typing.py. The second half will be the equivalent machinery in genericaliasobject.c, but I'll do that in a separate PR, since I guess different people will want to review it.

[AlexWaygood]

(I think this probably does need a news entry, FWIW, since it wasn't in beta 1, and I assume we'll be backporting.)

[mrahtz]

I've just realised; TODO @mrahtz What happens if there's more than one unpacked arbitrary-length tuple in the args list?

[mrahtz]

I've just realised; TODO @mrahtz What happens if there's more than one unpacked arbitrary-length tuple in the args list?

Thanks past me! Added a test for this situation.

[mrahtz]

Closing in favour of Serhiy's: #93330