Use Python 3 for test/demo code

Forwarded: not-needed

Gbp-Pq: Topic debian
Gbp-Pq: Name Use-Python-3-for-test-demo-code.patch

demos/userns-block-fd.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import os, select, subprocess, sys, json
 

tests/test-run.sh

@@ -226,7 +226,7 @@ fi
 # Test --die-with-parent
 
 cat >lockf-n.py <<EOF
-#!/usr/bin/env python
+#!/usr/bin/env python3
 import struct,fcntl,sys
 path = sys.argv[1]
 if sys.argv[2] == 'wait':