cdef _NONE, it shows up in some hot paths.

src/gevent/__waiter.pxd

@@ -6,6 +6,7 @@ cdef ConcurrentObjectUseError
 from gevent.__hub_local cimport get_hub_noargs as get_hub
 
 cdef bint _greenlet_imported
+cdef _NONE
 
 cdef extern from "greenlet/greenlet.h":
 
@@ -27,7 +28,6 @@ cdef inline void greenlet_init():
         PyGreenlet_Import()
         _greenlet_imported = True
 
-
 cdef class Waiter:
     cdef readonly hub
     cdef readonly greenlet