allow running 'webis.py' from multiple python versions

webis.py

-#!/usr/bin/env python3
-# -*- coding: utf-8 -*-
-"""The main webis command.
+#!/bin/sh
+# -*- coding: utf-8 ; mode: python -*-
+"true" '''\'
+# try running as python3, if that fails fall back to (any) python
+command -v python3 > /dev/null
+if [ $? -eq 0 ]; then
+    exec env python3 "$0" "$@"
+fi
+command -v python > /dev/null
+if [ $? -eq 0 ]; then
+    exec env python "$0" "$@"
+else
+    echo -e "\033[91m[ERROR] Install Python and try again!" 1>&2
+    exit 1
+fi
+'''
+__doc__ = """The main webis command.
 
 Copyright webis.de 2015-today
-Authors: Steve Göring, Martin Potthast
+Authors: Janek Bevendorff, Steve Göring, Martin Potthast, Michael Völske
 """
 
 import argparse