fix _abc.c compile error on Cygwin (GH-8445)

diff --git a/Modules/_abc.c b/Modules/_abc.c
index 36c1757..1fbf3a8 100644
--- a/Modules/_abc.c
+++ b/Modules/_abc.c
@@ -66,7 +66,7 @@
 "Internal state held by ABC machinery.");
 
 static PyTypeObject _abc_data_type = {
-    PyVarObject_HEAD_INIT(&PyType_Type, 0)
+    PyVarObject_HEAD_INIT(NULL, 0)
     "_abc_data",                        /*tp_name*/
     sizeof(_abc_data),                  /*tp_basicsize*/
     .tp_dealloc = (destructor)abc_data_dealloc,