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/src/gsm/Makefile.am b/src/gsm/Makefile.am
index f13ba9d..4a1faca 100644
--- a/src/gsm/Makefile.am
+++ b/src/gsm/Makefile.am
@@ -49,6 +49,6 @@
 
 # Convolutional codes generation
 gsm0503_conv.c: $(top_srcdir)/utils/conv_gen.py $(top_srcdir)/utils/conv_codes_gsm.py
-	$(AM_V_GEN)python $(top_srcdir)/utils/conv_gen.py gen_codes gsm
+	$(AM_V_GEN)python3 $(top_srcdir)/utils/conv_gen.py gen_codes gsm
 
 CLEANFILES = gsm0503_conv.c