Yes, code usually doesn't fall from the sky with a new CPython release. If something wants to make use of this ABC, it still has to find ways to also work with older CPythons. What would be a good fallback? A backport on PyPI?

I wouldn't even mind shipping this class with Cython and dropping it right into "collections.abc" if it's missing. Cython already contains lots of compatibility code, and we need to patch *something* in all current CPythons either way. Then your code snippet would be the right thing to do for user code (with the twist that Py2.x doesn't have "collections.abc"...).