test: fix Python 3.14 datetime deprecation

[fry69]

fix #669

This tiny patch fixes the Python 3.14 datetime deprecation warning while maintaining expected output format (without timezone information).

It should be compatible with Python 3.10 and up, but I have not tested this explicitly.

Update: The 2nd commit adds a suppression for another Python 3.14 related warning.

Pytest now runs completely without warnings on Python 3.14:

$ pytest
=== test session starts ===
platform darwin -- Python 3.14.0, pytest-8.4.2, pluggy-1.6.0
[...]
=== 1001 passed, 16 skipped in 5.94s ===

Generated explanation below:

---

Added a pytest marker to the test_dateparse_errors function:

@pytest.mark.filterwarnings("ignore::pytest.PytestUnraisableExceptionWarning")
def test_dateparse_errors(fresh_db, fn, errors):
    # ... test code ...

This decorator tells pytest to ignore PytestUnraisableExceptionWarning warnings for this specific test. This is the correct approach because:

1. The warnings are informational, not errors - The exceptions ARE being properly handled (they result in OperationalError as expected)
2. This is expected behavior - When SQLite user-defined functions raise exceptions, SQLite converts them to OperationalError, which the test correctly catches
3. Python 3.14 is just more strict - The new warning system flags these as "unraisable" even though they're handled correctly
4. Test-specific solution - Only the tests that intentionally trigger exceptions need this filter

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📚 Documentation preview 📚: https://sqlite-utils--671.org.readthedocs.build/en/671/