Add stub for xml.etree.ElementTree.indent (#5178)

stdlib/xml/etree/ElementTree.pyi

@@ -308,6 +308,10 @@ else:
     def tostringlist(element: Element, encoding: Optional[str] = ..., method: Optional[str] = ...) -> List[bytes]: ...
 
 def dump(elem: Element) -> None: ...
+
+if sys.version_info >= (3, 9):
+    def indent(tree: Union[Element, ElementTree], space: str = ..., level: int = ...) -> None: ...
+
 def parse(source: _file_or_filename, parser: Optional[XMLParser] = ...) -> ElementTree: ...
 def iterparse(
     source: _file_or_filename, events: Optional[Sequence[str]] = ..., parser: Optional[XMLParser] = ...