objstrunicode: Re-add buffer protocol back for now, required for io.StringIO.
diff --git a/py/objstrunicode.c b/py/objstrunicode.c
index 8fbe815..e6ca35a 100644
--- a/py/objstrunicode.c
+++ b/py/objstrunicode.c
@@ -313,7 +313,7 @@
     .binary_op = str_binary_op,
     .subscr = str_subscr,
     .getiter = mp_obj_new_str_iterator,
-//    .buffer_p = { .get_buffer = str_get_buffer },
+    .buffer_p = { .get_buffer = str_get_buffer },
     .locals_dict = (mp_obj_t)&str_locals_dict,
 };