Move the callback property to watcher.

src/gevent/libev/corecext.ppyx

@@ -669,15 +669,6 @@ cdef public class callback [object PyGeventCallbackObject, type PyGeventCallback
     cdef libev.ev_##TYPE _watcher                                                       \
                                                                                         \
                                                                                         \
-    @property \
-    def callback(self):                                                                  \
-        return self._callback                                                       \
-    \
-    @callback.setter \
-    def callback(self, object callback): \
-        if not PyCallable_Check(<PyObjectPtr>callback) and callback is not None:    \
-           raise TypeError("Expected callable, not %r" % (callback, ))             \
-        self._callback = callback                                                   \
                                                                                         \
     def stop(self):                                                                     \
         _check_loop(self.loop)                                                          \
@@ -784,6 +775,15 @@ cdef public class watcher [object PyGeventWatcherObject, type PyGeventWatcher_Ty
                libev.ev_unref(self.loop._ptr)
                self._flags |= 2
 
+    @property
+    def callback(self):
+        return self._callback
+
+    @callback.setter
+    def callback(self, object callback):
+        if callback is not None and not callable(callback):
+           raise TypeError("Expected callable, not %r" % (callback, ))
+        self._callback = callback
 
     def __repr__(self):
         if Py_ReprEnter(<PyObjectPtr>self) != 0: