tests: enable pytype checking on `mercurial/interfaces` package I might regret this, but the files currently scan OK, with the exception of one place where an `attribute-error` is now disabled. We should fix this somehow, but the rest of this package needs to be converted to Protocol classes first. I'm enabling checking on this now because I got to the tail end of the series converting this to Protocol classes, and suddenly one of the commits is causing pytype to fail with an error message that doesn't really jive with the few code changes in it. Then when I ran `pytype-single` on it to get a result faster, I got a host of other unexpected errors that were likely from earlier in the series. So first things first, strictly check the content of these files going forward.