Drop "python-dateutil" dependency (Python 3.12 compatibility)

[zegl]

Python 3.12 has deprecated datetime.datetime.utcfromtimestamp, and will be removed.

python-posthog depends on this deprecated function via the python-dateutil library.

The usage of dateutil in this package does not seem to very widespread. I'd consider removing the dependency and relying on the native Python datetime libraries instead.

.venv/lib/python3.12/site-packages/dateutil/tz/tz.py:37
  /.venv/lib/python3.12/site-packages/dateutil/tz/tz.py:37: DeprecationWarning: datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.fromtimestamp(timestamp, datetime.UTC).
    EPOCH = datetime.datetime.utcfromtimestamp(0)