Default to Python 3

In support of #8.

Right now I've just installed it and I'm trying to understand the right way to default to python3. There is the option of symlinking both `python` and `pip`.
If you're concerned about other processes being able to see it in environment variables, I'm not sure this will help, because you can find the command line arguments of other processes (at least on regular systems - I assume the same is true on Docker). That's a separate question from porting to Python 3, though).