Stop using datetime.utcnow() in tests

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

datetime.utcnow() is deprecated for Python 3.12+, and raises a warning. Since warnings are treated as errors, this results in test failures. Since utcnow calls are done by the SQLAlchemy mapping machinery, we need to use a callable.

Fixes #1303

Checklist:

• [x] Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• [x] Add or update relevant docs, in the docs folder and in code.
• [x] Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• [x] Add .. versionchanged:: entries in any relevant code docs.
• [x] Run pre-commit hooks and fix any issues.
• [x] Run pytest and tox, no tests failed.

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

I have addressed both comments, thanks!

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

@davidism Thanks, I've reverted that bit.

[davidism]

Sorry, completely forgot about this PR and ended up fixing it myself when doing some updates. Thanks for working on it!