fix: issue2551278 - datetime.datetime.utcnow deprecation. We now use the timezone aware utc dates for python 3.11+. But we have to make all the rest of the dates (datetime.min, unix epoch date) timezon aware so we can subtract them. Also need to marshall/unmarshall timezone aware iso formatted date strings.

License
=======

Roundup is Free Software under the following license (from the file
COPYING.txt in the software distribution):

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   <div class="file_insert">

.. include:: ../COPYING.txt

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   </div>