Another question: is it ok if Cython implements and uses the "tp_as_async" slot in all Py3.x versions (3.2+)? It shouldn't hurt, but it would speed up async/await in Cython at least under Py3.x. Only Py2.6/7 would then have to resort to calling "__await__()" etc. at the Python level.

One drawback is that Py<3.5 currently (needlessly) checks that "tp_reserved" and "tp_richcompare" are both implemented, but that can be worked around by also implementing the latter...