Revert "Use python 3 for utilities"

This reverts commit 76c6c50405c6cbb1d08bdd7b5d27c657fa5d38b6, which broke the obs builds. I'm really starting to get annoyed by ongoing python related breakage without ever fixing any bugs!

Change-Id: I4d76e897d4f746ff9ea4e06f2efc708a12cc2944
diff --git a/utils/conv_codes_gsm.py b/utils/conv_codes_gsm.py
index 72681e7..eeed4fd 100644
--- a/utils/conv_codes_gsm.py
+++ b/utils/conv_codes_gsm.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/python2
 # -*- coding: utf-8 -*-
 from conv_gen import ConvolutionalCode
 from conv_gen import poly