Remove deprecated naive datetime warnings

[hornc]

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).

These warnings were appearing ~4000 times in test results, datetime.utcnow() is deprecated in Python 3.12
For some background: https://blog.miguelgrinberg.com/post/it-s-time-for-a-change-datetime-utcnow-is-now-deprecated

This fixes the warnings caused by code in this repo. I think it doesn't introduce any new problems, but utcnow() was used in a range of places, having others review these changes would be helpful, in case there is a subtle impact from format changes I have missed.

BEFORE:

2162 passed, 9 skipped, 9 xfailed, 4891 warnings in 6.21s

AFTER:

2162 passed, 9 skipped, 9 xfailed, 20 warnings in 6.18s

Remaining utcnow() deprecation warnings are in external repos: Infogami and webpy:

openlibrary/plugins/admin/tests/test_code.py::TestRevertAllUserEdits::test_no_edits
openlibrary/plugins/admin/tests/test_code.py::TestRevertAllUserEdits::test_deletes_spam_works
openlibrary/plugins/admin/tests/test_code.py::TestRevertAllUserEdits::test_deletes_spam_works
openlibrary/plugins/admin/tests/test_code.py::TestRevertAllUserEdits::test_reverts_spam_edits
openlibrary/plugins/admin/tests/test_code.py::TestRevertAllUserEdits::test_reverts_spam_edits
openlibrary/plugins/admin/tests/test_code.py::TestRevertAllUserEdits::test_does_not_undelete
openlibrary/plugins/admin/tests/test_code.py::TestRevertAllUserEdits::test_two_spammy_editors
openlibrary/plugins/admin/tests/test_code.py::TestRevertAllUserEdits::test_two_spammy_editors
  /openlibrary/infogami/infobase/account.py:87: 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).
    now = datetime.utcnow()

openlibrary/tests/core/test_db.py::TestCheckIns::test_update_event_date
openlibrary/tests/core/test_db.py::TestYearlyReadingGoals::test_update_current_count
openlibrary/tests/core/test_db.py::TestYearlyReadingGoals::test_update_target
openlibrary/tests/core/test_imports.py::TestBatchItem::test_add_items_legacy
  /home/openlibrary/.local/lib/python3.12/site-packages/web/db.py:487: DeprecationWarning: The default timestamp converter is deprecated as of Python 3.12; see the sqlite3 documentation for suggested replacement recipes
    row = self.cursor.fetchone()

Closes #

Technical

Testing

Screenshot

Stakeholders

[scottbarnes]

Looks good to me. Also, I appreciate the timezone being added to the /status page. :)