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().