gh-105927: Add _PyWeakref_IS_DEAD() function (#105992)

* Add _PyWeakref_IS_DEAD() internal function.
* Modify is_dead_weakref() of Modules/_weakref.c and
  _pysqlite_drop_unused_cursor_references() to replace
  PyWeakref_GET_OBJECT() with _PyWeakref_IS_DEAD().
* Replace "int i" with "Py_ssize_t i" to iterate on cursors
  in _pysqlite_drop_unused_cursor_references().

Include/internal/pycore_weakref.h

@@ -33,6 +33,19 @@ static inline PyObject* _PyWeakref_GET_REF(PyObject *ref_obj) {
     return Py_NewRef(obj);
 }
 
+static inline int _PyWeakref_IS_DEAD(PyObject *ref_obj) {
+    assert(PyWeakref_Check(ref_obj));
+    PyWeakReference *ref = _Py_CAST(PyWeakReference*, ref_obj);
+    PyObject *obj = ref->wr_object;
+    if (obj == Py_None) {
+        // clear_weakref() was called
+        return 1;
+    }
+
+    // See _PyWeakref_GET_REF() for the rationale of this test
+    return (Py_REFCNT(obj) == 0);
+}
+
 #ifdef __cplusplus
 }
 #endif