Change all python files to use Python3

contrib/macdeploy/custom_dsstore.py

@@ -1,8 +1,7 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # Copyright (c) 2013-2016 The Bitcoin Core developers
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
-from __future__ import division,print_function,unicode_literals
 import biplist
 from ds_store import DSStore
 from mac_alias import Alias