utils: add function now

[utnapischtim]

No description provided.

[slint]

IMHO I would prefer that we find a more "SQLAlchemy way" to avoid this footgun:

• It's going to be difficult to teach newcomers and (unfortunately, manually) enforce thoughout the codebase doing from invenio_db import now everytime we store UTC/non-TZ-aware datetimes in the DB.
• It's going to be difficult to catch datetime-related bugs in tests, since often we just "ignore" the values of created/updated fields in responses and focus more on "data content" (which is not a bad thing when applied consiously).
• I would be more in favour towards finding some config/setting/param in SQLAlchemy datetime columns or even defining a new custom DateTime column type that "does the right thing"™️ on (de)serialization which we can then gradually replace throughout the codebase in models that:
  • Doesn't affect Alembic migrations (e.g. column types in Postgres are still TIMESTAMP WITHOUT TIME ZONE)
  • Makes sure that Python datetime objects with the UTC timezone, get transformed accordingly
  • Explicitly complains/errors when non-UTC timezone-aware datetimes are passed, so that it's not the humans/reviewers that have to check, but the code does it instead.

There's nothing to be done at the moment, but this should be discussed a bit in more detail, so we make a sound decision that we apply.

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On a side note, I think we've never explicitly wrote this anywhere, but the common practice throughout Invenio is to store/persist UTC times and convert to/from local timezone-aware times in the UI. I don't know where to log this "decision", but this post is a good supplementary read, from which I get the feeling that for us storing UTC is better.

[utnapischtim]

superseded by #179