Replace deprecated serpapi_pagination.next_link with next

Ref: serpapi/SerpApi#1656

Author: ilyazub

serpapi/models.py

@@ -51,7 +51,7 @@ def next_page_url(self):
         serpapi_pagination = self.data.get("serpapi_pagination")
 
         if serpapi_pagination:
-            return serpapi_pagination.get("next_link")
+            return serpapi_pagination.get("next")
 
     def next_page(self):
         """Return the next page of results, if any."""