twister: Refactor python module to follow PEP8 rules

Incremental refactoring.
Fix PEP8 issues to make ruff green for config_parser.py module.
Add __init__.py to twister directory to make it a proper python package,
and make modules importable.

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