btsocket: Replace the shebang of setup.py with py3

Replace the shebang of setup.py with py3 to avoid the warning.

BUG=b:219620510
TEST=None

Change-Id: Icad31257b2815fc6f424fe4180d72ffe2ea27a2f
(cherry picked from commit 7dfb270c66a93e8680c103c22122e5518803ca4d)
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/btsocket/+/4443857
Tested-by: Chris Gerber <gerb@google.com>
Auto-Submit: Chris Gerber <gerb@google.com>
Reviewed-by: Yu-Hsuan Hsu <yuhsuan@chromium.org>
Commit-Queue: Yu-Hsuan Hsu <yuhsuan@chromium.org>
diff --git a/setup.py b/setup.py
index 509114d..9663bc5 100755
--- a/setup.py
+++ b/setup.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2
+#!/usr/bin/env python3
 # Copyright (c) 2013 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.