imaplib: append returns a tuple (status, [data])

According to the docstring for append:
```
(typ, [data]) = <instance>.append(mailbox, flags, date_time, message)
```
[v3.0](https://github.com/python/cpython/blob/v3.0/Lib/imaplib.py#L313)
[v3.14](https://github.com/python/cpython/blob/v3.14.0/Lib/imaplib.py#L483)

Further corroborated that this is a tuple of some sort because append
returns the result of `self._simple_command(...)`, which returns the
result of `self._command_complete(...)`, which returns `typ, data`.

Author: savanto

stdlib/imaplib.pyi

@@ -61,7 +61,7 @@ class IMAP4:
     def socket(self) -> _socket: ...
     def recent(self) -> _CommandResults: ...
     def response(self, code: str) -> _CommandResults: ...
-    def append(self, mailbox: str, flags: str, date_time: str, message: ReadableBuffer) -> str: ...
+    def append(self, mailbox: str, flags: str, date_time: str, message: ReadableBuffer) -> tuple[str, list[bytes]]: ...
     def authenticate(self, mechanism: str, authobject: Callable[[bytes], bytes | None]) -> tuple[str, str]: ...
     def capability(self) -> _CommandResults: ...
     def check(self) -> _CommandResults: ...