I don't think we need the types.GenericAlias(int | list[T], str)
workaround. GenericAlias already has code for extracting __parameters__ and
implementing __getitem__. Someone would need to refactor and reuse them for
Union too. That should be enough to trick GenericAlias to treat Union as
another GenericAlias(I don't remember if it checks for __origin__ as well)
and cover other edge cases as well.

@Yurii do you still plan to take this? If not, I'll start working on
something later today.