Allow type parameters without default values to follow those with default values in some situations

[AlexWaygood]

This is a backport of python/cpython#118774. The implementation feels unbelievably hacky (we're looking up the value of a local variable in frame two levels up the call stack), but I can't see another way of doing it. I initially tried an approach where we monkeypatched _GenericAlias.__init__, but this didn't work: see python/cpython#118774 (comment).

[JelleZijlstra]

Working on the merge conflict. I think I wouldn't want to use a _getframe-based hack, but I'll have to think of an alternative.

[AlexWaygood]

I think I wouldn't want to use a _getframe-based hack, but I'll have to think of an alternative.

Yeah. I think we definitely need to make some change to our current implementation at typing_extensions, since currently we're strictly more prohibitive than the CPython implementation, and that's not great. If we really don't like this hack, though, I think it would be okay (though not ideal) to be strictly more lenient than the CPython implementation -- simply not check the order of the parameters under any circumstances.

[JelleZijlstra]

Thinking about it more, I think this is OK. It's well localized and only affects old versions of typing.