gh-138720: Make Buffered closed check match flush (GH-138724)

In `_io__Buffered_flush_impl` the macro `CHECK_CLOSED` is used to check
the `buffered*` is in a good state to be flushed. That differs slightly
from `buffered_closed`.

In some cases, that difference would result in `close()` thinking the
file needed to be flushed and closed while `flush()` thought the file
was already closed.

This could happen during GC and would result in an unraisable exception.

Modules/_io/bufferedio.c

@@ -553,8 +553,8 @@ _io__Buffered_close_impl(buffered *self)
     if (!ENTER_BUFFERED(self)) {
         return NULL;
     }
-
-    r = buffered_closed(self);
+    /* gh-138720: Use IS_CLOSED to match flush CHECK_CLOSED. */
+    r = IS_CLOSED(self);
     if (r < 0)
         goto end;
     if (r > 0) {