test warning with python3.12

[asarubbo]

By packaging 3.5.0 on Gentoo Linux I see the following warning that come from test with python-3.12:

=============================== warnings summary ===============================
pgactivity/utils.py::pgactivity.utils.csv_write
  /var/tmp/portage/dev-db/pg_activity-3.5.0/work/pg_activity-3.5.0/pgactivity/utils.py:283: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).
    dt = datetime.utcnow().strftime("%Y-%m-%dT%H:%m:%SZ")

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html

[dlax]

Indeed, the warnings also show up in our CI: https://github.com/dalibo/pg_activity/actions/runs/8112276447/job/22173151102#step:6:109; we should turn them into errors.

Thanks for the report.