I doubt effbot will respond. :-)

I think the first order of business is to add the same FutureWarning to the C version of the code, and to reset the clock on the deprecation. 

In terms of the final stages, why wouldn't we want it to just always raise? Just because there's so much code that might break?

It shouldn't be a SyntaxWarning, since this is most definitely not a syntax issue. I'd be okay with RuntimeError (or RuntimeWarning if we decide to keep the old outcome with a warning forever).

(Huh, does FutureWarning count as a deprecation?)