Upgrade dateutil to treat epoch as naive

[janbrasna]

One of the dependencies that are constrained to its minor semver range is:

thunderbird-website/requirements-dev.txt

Line 27 in 5a2911c

python-dateutil~=2.8.2

To get rid of the TZ/naive deprecation warnings AFAICT a 2.9.x upgrade would be prudent:

• https://github.com/dateutil/dateutil/releases/tag/2.9.0

[janbrasna]

It's actually only used in one place and it's just the dependency itself:

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).

  /Library/Frameworks/Python.framework/Versions/3.14/lib/python3.14/site-packages/dateutil/tz/tz.py:37:

    EPOCH = datetime.datetime.utcfromtimestamp(0)

so there should be no more occurrences beyond this.