Update _middlewares.py

Fix The annoyning Deprecation issue
by just using get_slot_key instead of _get_slot_key and remove the second param

scrapy_zyte_api/_middlewares.py

@@ -31,7 +31,7 @@ def slot_request(self, request, spider, force=False):
             return
 
         downloader = self._crawler.engine.downloader
-        slot_id = downloader._get_slot_key(request, spider)
+        slot_id = downloader.get_slot_key(request)
         if not isinstance(slot_id, str) or not slot_id.startswith(self._slot_prefix):
             slot_id = f"{self._slot_prefix}{slot_id}"
             request.meta["download_slot"] = slot_id