dss.py: enforce python3

Some systems come with python2 as default interpreter.

Signed-off-by: Ralf Ramsauer <ralf@binary-kitchen.de>

dss.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 from flask import Flask, render_template, redirect, url_for, session
 from flask_wtf import Form