Python 3.12 changes cause test failures due to deprecation warning

[s-t-e-v-e-n-k]

Running the test suite under Python 3.12 has multiple failures that all stem from:

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).

Since warnings are treated as errors, this causes a failure.