logging.handlers: Add BufferingHandler.buffer (#3402)

Signed-off-by: Nikola Forró <nforro@redhat.com>

stdlib/2and3/logging/handlers.pyi

@@ -170,6 +170,7 @@ class SMTPHandler(Handler):
 
 
 class BufferingHandler(Handler):
+    buffer: List[LogRecord]
     def __init__(self, capacity: int) -> None: ...
     def shouldFlush(self, record: LogRecord) -> bool: ...