py: Change type signature of builtin funs that take variable or kw args.

With this patch the n_args parameter is changed type from mp_uint_t to
size_t.
diff --git a/py/objstringio.c b/py/objstringio.c
index d046cdd..357320c 100644
--- a/py/objstringio.c
+++ b/py/objstringio.c
@@ -114,7 +114,7 @@
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(stringio_close_obj, stringio_close);
 
-STATIC mp_obj_t stringio___exit__(mp_uint_t n_args, const mp_obj_t *args) {
+STATIC mp_obj_t stringio___exit__(size_t n_args, const mp_obj_t *args) {
     (void)n_args;
     return stringio_close(args[0]);
 }