extmod/modbtree: Retain reference to underlying stream so it's not GC'd.

For ports that have a system malloc which is not garbage collected (eg
unix, esp32), the stream object for the DB must be retained separately to
prevent it from being reclaimed by the MicroPython GC (because the
berkeley-db library uses malloc to allocate the DB structure which stores
the only reference to the stream).

Although in some cases the user code will explicitly retain a reference to
the underlying stream because it needs to call close() on it, this is not
always the case, eg in cases where the DB is intended to live forever.

Fixes issue #5940.

examples/natmod/btree/btree_c.c

@@ -115,7 +115,7 @@ STATIC mp_obj_t btree_open(size_t n_args, const mp_obj_t *args) {
         mp_raise_OSError(native_errno);
     }
 
-    return MP_OBJ_FROM_PTR(btree_new(db));
+    return MP_OBJ_FROM_PTR(btree_new(db, args[0]));
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(btree_open_obj, 5, 5, btree_open);