Change all python files to use Python3

share/qt/extract_strings_qt.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # Copyright (c) 2012-2017 The Bitcoin Core developers
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
@@ -6,7 +6,6 @@
 Extract _("...") strings for translation and convert to Qt stringdefs so that
 they can be picked up by Qt linguist.
 '''
-from __future__ import division,print_function,unicode_literals
 from subprocess import Popen, PIPE
 import operator
 import os