gh-130141: mark _SelectorTransport as closing during del

Author: lunixbochs

Lib/asyncio/selector_events.py

@@ -871,6 +871,8 @@ def close(self):
     def __del__(self, _warn=warnings.warn):
         if self._sock is not None:
             _warn(f"unclosed transport {self!r}", ResourceWarning, source=self)
+            self._closing = True
+            self._buffer.clear()
             self._sock.close()
             if self._server is not None:
                 self._server._detach(self)