title: Execute current Python script in Python 3
body: <p>I have a python program which I have made work in both Python 2 and 3, and it has more functionality in Python 3 (using new Python 3 features).</p>
  
  <p>My script currently starts <code>#!/usr/bin/env python</code>, as that seems to be the mostly likely name for a python executable. However, what I'd like to do is "if python3 exists, use that, if not use python".</p>
  
  <p>I would prefer not to have to distribute multiple files / and extra script (at present my program is a single distributed python file).</p>
  
  <p>Is there an easy way to run the current script in python3, if it exists?</p>