Use python 3 for utilities

There're no python2-specific code in there so we can switch right away
without waiting till 2020 for python 2 deprecation.

Related: OS#2819
Change-Id: I8d34aed124b00c5dd2ab1bcc84bbfa8c620282cc

debian/control

@@ -15,7 +15,7 @@ Build-Depends: debhelper (>= 9),
                libpcsclite-dev,
                pkg-config,
                libtalloc-dev,
-               python (>= 2.7.6)
+               python3
 Standards-Version: 3.9.8
 Vcs-Git: git://git.osmocom.org/libosmocore.git
 Vcs-Browser: http://git.osmocom.org/gitweb?p=libosmocore.git;a=summary

utils/conv_codes_gsm.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python2
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 from conv_gen import ConvolutionalCode
 from conv_gen import poly

utils/conv_gen.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python2
+#!/usr/bin/env python3
 
 mod_license = """
 /*