Change shebang to use env for Python

As a general good practice, it's worth using an env shebang.
What's more, it was already brought up in #2.

Author: pr0way

perplexity.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 import logging
 import argparse
 import os