Use PEP 688 (#10225)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

stdlib/imaplib.pyi

@@ -1,7 +1,7 @@
 import subprocess
 import sys
 import time
-from _typeshed import ReadableBuffer, _BufferWithLen
+from _typeshed import ReadableBuffer, SizedBuffer
 from builtins import list as _list  # conflicts with a method named "list"
 from collections.abc import Callable
 from datetime import datetime
@@ -155,7 +155,7 @@ class _Authenticator:
     def __init__(self, mechinst: Callable[[bytes], bytes | bytearray | memoryview | str | None]) -> None: ...
     def process(self, data: str) -> str: ...
     def encode(self, inp: bytes | bytearray | memoryview) -> str: ...
-    def decode(self, inp: str | _BufferWithLen) -> bytes: ...
+    def decode(self, inp: str | SizedBuffer) -> bytes: ...
 
 def Internaldate2tuple(resp: ReadableBuffer) -> time.struct_time | None: ...
 def Int2AP(num: SupportsAbs[SupportsInt]) -> bytes: ...