sqlite3 default datetime adapter is deprecated as of Python 3.12

[luator]

When running the tests with Python 3.12, I get the following warning:

  /home/felixwidmaier/ws/comprl/comprl/comprl/server/data/sql_backend.py:64: DeprecationWarning: The default datetime adapter is deprecated as of Python 3.12; see the sqlite3 documentation for suggested replacement recipes
    self.cursor.execute(

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html

Would probably be good to resolve this rather soon to make it future proof (and to silence the warning).

[luator]

Has been fixed (I think by #135).