Call PyBaseObject_Type.tp_new instead of tp_alloc

This allows object.__new__ to check greenlet subclasses for
being abstract and raise an exception in that case.

greenlet.c

@@ -130,6 +130,8 @@ static PyObject* ts_event_throw;
 #endif
 static PyObject* PyExc_GreenletError;
 static PyObject* PyExc_GreenletExit;
+static PyObject* ts_empty_tuple;
+static PyObject* ts_empty_dict;
 
 #if GREENLET_USE_GC
 #define GREENLET_GC_FLAGS Py_TPFLAGS_HAVE_GC
@@ -825,7 +827,7 @@ static PyObject* green_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 	if (!STATE_OK)
 		return NULL;
 
-	o = type->tp_alloc(type, 0);
+	o = PyBaseObject_Type.tp_new(type, ts_empty_tuple, ts_empty_dict);
 	if (o != NULL) {
 		Py_INCREF(ts_current);
 		((PyGreenlet*) o)->parent = ts_current;
@@ -1634,6 +1636,18 @@ initgreenlet(void)
 		INITERROR;
 	}
 
+	ts_empty_tuple = PyTuple_New(0);
+	if (ts_empty_tuple == NULL)
+	{
+		INITERROR;
+	}
+
+	ts_empty_dict = PyDict_New();
+	if (ts_empty_dict == NULL)
+	{
+		INITERROR;
+	}
+
 	ts_current = green_create_main();
 	if (ts_current == NULL)
 	{

tests/test_greenlet.py

@@ -6,6 +6,12 @@
 
 from greenlet import greenlet
 
+try:
+    from abc import ABCMeta, abstractmethod
+except ImportError:
+    ABCMeta = None
+    abstractmethod = None
+
 
 class SomeError(Exception):
     pass
@@ -462,3 +468,20 @@ def switchapply():
                 apply(greenlet.getcurrent().parent.switch, args, kwargs)
             g = greenlet(switchapply)
             self.assertEqual(g.switch(), kwargs)
+
+    if ABCMeta is not None and abstractmethod is not None:
+        def test_abstract_subclasses(self):
+            AbstractSubclass = ABCMeta(
+                'AbstractSubclass',
+                (greenlet,),
+                {'run': abstractmethod(lambda self: None)})
+
+            class BadSubclass(AbstractSubclass):
+                pass
+
+            class GoodSubclass(AbstractSubclass):
+                def run(self):
+                    pass
+
+            GoodSubclass() # should not raise
+            self.assertRaises(TypeError, BadSubclass)