Change env to python3 instead of python3.6

This change allows users with later versions of python3 to run the
program without having to make sure they have specifically python3.6
installed.

trade.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3.6
+#!/usr/bin/env python3
 # --------------------------------------------------------------------
 # Copyright (C) Oliver 'kfsone' Smith 2014 <[email protected]>:
 # Copyright (C) Bernd 'Gazelle' Gollesch 2016, 2017