Fix wrong type hints for SyndicationFeed methods

Django's documentation mentions that the handles is of type
xml.sax.saxutils.XMLGenerator. However, it is actually of type
django.utils.xmlutils.SimplerXMLGenerator, which is a subclass of the
former.

Methods like `addQuickElement` (which are used in django's own
implementation of add_root_elements) is actually defined for
SimplerXMLGenerator, so this seems like the best fit (since subclasses
will generally rely on this method).

django-stubs/utils/feedgenerator.pyi

@@ -1,6 +1,7 @@
 import datetime
 from typing import Any
 from xml.sax import ContentHandler
+from django.utils.xmlutils import SimplerXMLGenerator
 
 from typing_extensions import TypeAlias
 
@@ -49,9 +50,13 @@ class SyndicationFeed:
     ) -> None: ...
     def num_items(self) -> int: ...
     def root_attributes(self) -> dict[Any, Any]: ...
-    def add_root_elements(self, handler: ContentHandler) -> None: ...
+    def add_root_elements(self, handler: SimplerXMLGenerator) -> None: ...
     def item_attributes(self, item: dict[str, Any]) -> dict[Any, Any]: ...
-    def add_item_elements(self, handler: ContentHandler, item: dict[str, Any]) -> None: ...
+    def add_item_elements(
+        self,
+        handler: SimplerXMLGenerator,
+        item: dict[str, Any],
+    ) -> None: ...
     def write(self, outfile: Any, encoding: Any) -> None: ...
     def writeString(self, encoding: str) -> str: ...
     def latest_post_date(self) -> datetime.datetime: ...