don't accept bytes in FileIO.write #7785
diff --git a/Modules/_io/fileio.c b/Modules/_io/fileio.c
index 37ddaeb..f02fe8a 100644
--- a/Modules/_io/fileio.c
+++ b/Modules/_io/fileio.c
@@ -648,7 +648,7 @@
 	if (!self->writable)
 		return err_mode("writing");
 
-	if (!PyArg_ParseTuple(args, "s*", &pbuf))
+	if (!PyArg_ParseTuple(args, "y*", &pbuf))
 		return NULL;
 
 	if (_PyVerify_fd(self->fd)) {