Fix typing for XMLPullParser.feed() (#7695)

Closes #7681

stdlib/xml/etree/ElementTree.pyi

@@ -319,7 +319,7 @@ def iterparse(
 
 class XMLPullParser:
     def __init__(self, events: Sequence[str] | None = ..., *, _parser: XMLParser | None = ...) -> None: ...
-    def feed(self, data: bytes) -> None: ...
+    def feed(self, data: str | bytes) -> None: ...
     def close(self) -> None: ...
     def read_events(self) -> Iterator[tuple[str, Element]]: ...