Make python action use vpython3

Author: iskakaushik

tools/fuchsia/.vpython renamed to tools/.vpython

@@ -1,10 +1,10 @@
-#!/usr/bin/env python
+#!/usr/bin/env vpython3
 #
 # Copyright 2021 The Flutter Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import os
 import sys
+import subprocess
 
-os.execv('/usr/bin/python3', ['python3'] + sys.argv[1:])
+subprocess.run(['vpython3'] + sys.argv[1:])