debian, utils: switch to python 3

Make build and external tests work with python3, so we can drop
the python2 dependency.

This should be merged shortly after osmo-python-tests was migrated to
python3, and the jenkins build slaves were (automatically) updated to
have the new osmo-python-tests installed.

Related: OS#2819
Depends: osmo-python-tests I3ffc3519bf6c22536a49dad7a966188ddad351a7
Change-Id: I84ef43f700e125c7a65f92347f12844e07e65655
diff --git a/utils/conv_codes_gsm.py b/utils/conv_codes_gsm.py
index ec77660..e189985 100644
--- a/utils/conv_codes_gsm.py
+++ b/utils/conv_codes_gsm.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 
 from conv_gen import ConvolutionalCode