MNT: Remove use of utcnow() and change to naive datetimes instead.

[greglucas]

utcnow() is deprecated since Python 3.12 in preference of now(UTC). However, here st_time actually returns a naive datetime, so doing now(UTC) returns a TypeError because you can't subtract a naive time from a timezone aware time. So, I think we should actually be using whatever the system is reporting for the timezone with just now().

https://docs.python.org/3/library/datetime.html#datetime.datetime.utcnow

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.50%. Comparing base (efa3faa) to head (b0803fd).
Report is 54 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1450      +/-   ##
==========================================
+ Coverage   96.40%   96.50%   +0.09%     
==========================================
  Files          20       20              
  Lines        1810     1830      +20     
==========================================
+ Hits         1745     1766      +21     
+ Misses         65       64       -1     

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[snowman2]

Thanks @greglucas 👍