In reviewing https://github.com/cython/cython/pull/3009, Jeroen pointed out that my symbol checking idea wouldn't actually work, since the preprocessor can only see preprocessor definitions, not compiler symbols. If we're going to rely on a preprocessor definition, it may as well be PY_VERSION_HEX, so I've updated the Cython PR accordingly.

That means we're back to either breaking Cython compatibility with CPython master until the release is cut, or else just bumping the release serial a little early. The duration of either state is now a lot shorter though, since the target release date for 3.8.0b2 is next week.