Add email.message.Message.__iter__ (#5960)

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

stdlib/email/message.pyi

@@ -28,6 +28,7 @@ class Message:
     def get_charset(self) -> _CharsetType: ...
     def __len__(self) -> int: ...
     def __contains__(self, name: str) -> bool: ...
+    def __iter__(self) -> Iterator[str]: ...
     def __getitem__(self, name: str) -> _HeaderType: ...
     def __setitem__(self, name: str, val: _HeaderType) -> None: ...
     def __delitem__(self, name: str) -> None: ...