Fix the deprecation status of the datetime library usage

[gridhead]

Here is what the logs look like.

test/test_tkts.py: 31 warnings
test/test_unit_gitlab.py: 1 warning
  /home/altruism/Projects/pagure-exporter/pagure_exporter/work/tkts.py:207: 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).
    mm=datetime.utcfromtimestamp(standard.cmtstime).strftime("%M"),