Stefan,

This patch should solve the problem with types.coroutine accepting only pure python generator functions.

The approach is, however, slightly different from what you've proposed.  Instead of having a wrapper class (delegating .throw, .send etc to a wrapped object), we now simply check if the returned value of the wrapped function is an instance of collections.abc.Coroutine.  Issue 24315 enables duck typing for coroutines, so if a cython-based coroutine implements all coroutine abstract methods, it will automatically pass types.coroutine.