stdlib: Add email.iterators._structure(msg)

stdlib/email/iterators.pyi

@@ -1,3 +1,4 @@
+from _typeshed import SupportsWrite
 from email.message import Message
 from typing import Iterator
 
@@ -6,3 +7,6 @@ __all__ = ["body_line_iterator", "typed_subpart_iterator", "walk"]
 def body_line_iterator(msg: Message, decode: bool = ...) -> Iterator[str]: ...
 def typed_subpart_iterator(msg: Message, maintype: str = ..., subtype: str | None = ...) -> Iterator[str]: ...
 def walk(self: Message) -> Iterator[Message]: ...
+
+# We include the seemingly private function because it is documented in the stdlib documentation.
+def _structure(msg: Message, fp: SupportsWrite[str] | None = ..., level: int = ..., include_default: bool = ...) -> None: ...