yaml importer uses incorrect timestamps when the server isn't in UTC

https://github.com/cms-dev/cms/blob/fcb70eedc4552892f216606ca2864f4a130867d3/cmscontrib/loaders/italy_yaml.py#L127-L131

the yaml importer creates aware datetime objects with the timezone set to utc. sqlalchemy doesn't seem to enjoy these:

>>> import cms.db
>>> from datetime import datetime, timezone
>>> session = cms.db.Session()
>>> contest = cms.db.Contest.get_from_id(1, session)
>>> contest.start = datetime(2025, 9, 28, tzinfo=timezone.utc)
>>> contest.start
datetime.datetime(2025, 9, 28, 0, 0, tzinfo=datetime.timezone.utc)
>>> session.commit()
>>> contest.start
datetime.datetime(2025, 9, 28, 3, 0)

short term fix: we need to convert these aware datetimes to naive before passing them to sqlalchemy. longer term fix: we should probably be using aware datetimes instead... and possibly upgrading our sqlalchemy (it already throws warnings about deprecated datetime methods)

Read cmscontrib/loaders/italy_yaml.py at lines 127-131 and reproduce the importer with the server in a non-UTC timezone. Check how the resulting datetime is persisted through SQLAlchemy, then verify that imported timestamps retain the intended value without timezone-related conversion or warnings.