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>