factory: Switch python2 to python3

Use the following command to modify:
```
ag "python2" -l | xargs -I{} sed -i 's/python2/python3/g' {}
```

BUG=chromium:999876
TEST=None

Change-Id: I19a5ced2f3bee0f26950f8948bbb9d8a07de46fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/factory/+/1991202
Tested-by: Yilin Yang (kerker) <kerker@chromium.org>
Commit-Queue: Yilin Yang (kerker) <kerker@chromium.org>
Reviewed-by: Yong Hong <yhong@chromium.org>
diff --git a/py/tools/regcode.py b/py/tools/regcode.py
index 3b686dc..bd17d23 100755
--- a/py/tools/regcode.py
+++ b/py/tools/regcode.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2
+#!/usr/bin/env python3
 #
 # Copyright 2014 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be