osmoappdesc.py: switch to python 3

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

Note that the external tests, which are using this file, are currently
not enabled in jenkins (OS#4317). However, I've manually verified that the
external tests work with this change.

Related: OS#2819
Depends: osmo-python-tests I3ffc3519bf6c22536a49dad7a966188ddad351a7
Change-Id: I19a996458745026cff60608710944e5ab76d8976
diff --git a/osmoappdesc.py b/osmoappdesc.py
index 38b18c0..331e6a9 100644
--- a/osmoappdesc.py
+++ b/osmoappdesc.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # (C) 2016 by Holger Hans Peter Freyther
 # This program is free software: you can redistribute it and/or modify