Don't hard code python path in scripts to support virtualenv

launch.py

-#!/usr/bin/python3 -OO
+#!/usr/bin/env python3
 
 import os
 import sys

scripts/gajim

-#!/usr/bin/python3 -OO
+#!/usr/bin/env python3
 ##
 ## Copyright (C) 2005 Yann Le Boulanger <asterix AT lagaule.org>
 ## Copyright (C) 2006 Dimitur Kirov <dkirov AT gmail.com>

scripts/gajim-history-manager

-#!/usr/bin/python3 -OO
+#!/usr/bin/env python3
 ##
 ## Copyright (C) 2005 Yann Le Boulanger <asterix AT lagaule.org>
 ## Copyright (C) 2006 Dimitur Kirov <dkirov AT gmail.com>

scripts/gajim-remote

-#!/usr/bin/python3 -OO
+#!/usr/bin/env python3
 ##
 ## Copyright (C) 2005 Yann Le Boulanger <asterix AT lagaule.org>
 ## Copyright (C) 2006 Dimitur Kirov <dkirov AT gmail.com>