twister: Fix failing tests on CI with Python 3.12

This change is removing some deprecation warnings
which for some reason causing failing tests with
Python 3.12 on CI #76877.

Also, it fixes warnings from pytest like:
PytestCollectionWarning: cannot collect test class 'TestPlan'
because it has a __init__ constructor
(from: scripts/tests/twister/test_testplan.py)

Signed-off-by: Lukasz Fundakowski <lukasz.fundakowski@nordicsemi.no>