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
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.