Fix python headers in some source files

These were missing the python2 header, so they went under my radar when
doing the switch go python3.

fdroidserver/common.py

@@ -1,4 +1,4 @@
-# -*- coding: utf-8 -*-
+#!/usr/bin/env python3
 #
 # common.py - part of the FDroid server tools
 # Copyright (C) 2010-13, Ciaran Gultnieks, ciaran@ciarang.com

fdroidserver/metadata.py

@@ -1,4 +1,4 @@
-# -*- coding: utf-8 -*-
+#!/usr/bin/env python3
 #
 # metadata.py - part of the FDroid server tools
 # Copyright (C) 2013, Ciaran Gultnieks, ciaran@ciarang.com

fdroidserver/net.py

@@ -1,4 +1,4 @@
-# -*- coding: utf-8 -*-
+#!/usr/bin/env python3
 #
 # net.py - part of the FDroid server tools
 # Copyright (C) 2015 Hans-Christoph Steiner <hans@eds.org>