[imaplib] Fix append args types (#15187)

* All arguments except `message` accept `None`.
* The `date_time` argument accepts all types that `Time2Internaldate` accepts.

Author: savanto

stdlib/imaplib.pyi

@@ -61,7 +61,9 @@ 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) -> tuple[str, _list[bytes]]: ...
+    def append(
+        self, mailbox: str | None, flags: str | None, date_time: _TimeLike | None, 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: ...
@@ -186,4 +188,7 @@ class _Authenticator:
 def Internaldate2tuple(resp: ReadableBuffer) -> time.struct_time | None: ...
 def Int2AP(num: SupportsAbs[SupportsInt]) -> bytes: ...
 def ParseFlags(resp: ReadableBuffer) -> tuple[bytes, ...]: ...
-def Time2Internaldate(date_time: float | time.struct_time | time._TimeTuple | datetime | str) -> str: ...
+
+_TimeLike: TypeAlias = float | time.struct_time | time._TimeTuple | datetime | str
+
+def Time2Internaldate(date_time: _TimeLike) -> str: ...