My bad. This was a typo caused by (badly) screwing up a merge conflict. I didn't spot it before pushing.

I'm going to rework the patch and build a new PR. My lack of git skills has got this one in a state where I'm not at all happy with it. But before I do, I'm going to try to write up my thoughts on the testing issues for pypa-dev because I'd like to thrash out the situation before I try another patch.