Support of Python3.10

This change includes required changes to make tests work in Python3.10.
It also enables tox running as part of stage-test, which was not included before.
For the moment, only flake8 and black envs will be checked. Some pylint
errors have been fixed, but a few remain unsolved. That's why pylint is
run, but its result is ignored.

Change-Id: I29105bf849eab8b47f92627a7eea1ede31b8e05b
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>
diff --git a/debian/control b/debian/control
index daa9f1a..1aa80b5 100644
--- a/debian/control
+++ b/debian/control
@@ -18,7 +18,7 @@
 Section: devel
 Priority: optional
 Maintainer: Gerardo Garcia <gerardo.garciadeblas@telefonica.com>
-Build-Depends: debhelper (>=9)
+Build-Depends: debhelper (>=9), debhelper-compat (=13)
 Standards-Version: 3.9.6
 Homepage: http://osm.etsi.org