This patch originated in-house but is not suitable for upstream.
 
--- meld-3.21.0/bin/meld	2020-04-19 03:11:05.000000000 +0000
+++ meld-3.21.0/bin/meld	2022-01-04 11:21:11.379866206 +0000
@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/python3.9
 
 # Copyright (C) 2002-2006 Stephen Kennedy <stevek@gnome.org>
 # Copyright (C) 2009-2014 Kai Willadsen <kai.willadsen@gmail.com>