Change GC refcount to Py_ssize_t.

Include/objimpl.h

@@ -250,7 +250,7 @@ typedef union _gc_head {
 	struct {
 		union _gc_head *gc_next;
 		union _gc_head *gc_prev;
-		int gc_refs;
+		Py_ssize_t gc_refs;
 	} gc;
 	long double dummy;  /* force worst-case alignment */
 } PyGC_Head;

Modules/gcmodule.c

@@ -303,7 +303,7 @@ visit_reachable(PyObject *op, PyGC_Head *reachable)
 {
 	if (PyObject_IS_GC(op)) {
 		PyGC_Head *gc = AS_GC(op);
-		const int gc_refs = gc->gc.gc_refs;
+		const Py_ssize_t gc_refs = gc->gc.gc_refs;
 
 		if (gc_refs == 0) {
 			/* This is in move_unreachable's 'young' list, but