_collect_type_vars should not collect Unpack objects itself

[Daraan]

Fixes: #471

• Backport of "_collect_type_vars" behaves wrong if Unpack is used on generic non-TypeVarTuple types #471

This PR fixes that these two cases should only yield T and not the unpack variable itself:

print(_collect_type_vars([Unpack[Movie[T]]]))
# (typing_extensions.Unpack[typing_extensions.Movie[~T]], ~T)

print(_collect_type_vars([Unpack[Tuple[T]]]))
# (typing_extensions.Unpack[typing.Tuple[~T]], ~T)

[JelleZijlstra]

This is looking good now, could you also add a changelog entry like Alex added on a few other PRs today?

[AlexWaygood]

Thanks!

[Daraan]

Thanks!

Thanks to you two as well!