[FIX] Remove deprecated utcnow

[teald]

This still fails some tests because of differences in 3.12. There's also an issue with times being truncated, which may be a formatting issue that cannot be addressed right now (see below).

This closes Issue #17

Truncated times issue

When running the 3.12 tests, get an initial failure at:

FAILED tests/unit/test_provenance.py::test_add_get_history - AssertionError: assert ('primitive', 'args', '2024-10-10T22:30:21.654469
+0', '2024-10-11T22:30:21.654469+0') == ('primitive', 'args', '2024-10-10T22:30:21.654469+00:00', '2024-10-11T22:30:21.654469+00:00')
  
  At index 2 diff: '2024-10-10T22:30:21.654469+0' != '2024-10-10T22:30:21.654469+00:00'
  
  Full diff:
    (
        'primitive',
        'args',
  -     '2024-10-10T22:30:21.654469+00:00',
  ?                                  ----
  +     '2024-10-10T22:30:21.654469+0',
  -     '2024-10-11T22:30:21.654469+00:00',
  ?                                  ----
  +     '2024-10-11T22:30:21.654469+0',
    )

This likely has to do with truncation happening that was not previously supported. I'm not sure if this is a limitation of FITS, astrodata, or something else at this point.

TODO

• Compare previous outputs directly.
  • Turn this into a test, to test for updates to any time formatting moving forward
  • Have single function for generating time strings? mostly affects astrodata/provenance.py