fix: sqlalchemy warning

[dextto]

Integration test results has an warning message according to SQLAlchemy on the tag ed2-c06-s04.
This is because of using deprecated module.

============================================================ warnings summary ============================================================
rentomatic/repository/postgres_objects.py:4
  /Users/dexter/src/rentomatic/rentomatic/repository/postgres_objects.py:4: MovedIn20Warning: The ``declarative_base()`` function is now available as sqlalchemy.orm.declarative_base(). (deprecated since: 2.0) (Background on SQLAlchemy 2.0 at: https://sqlalche.me/e/b8d9)
    Base = declarative_base()

[lgiordani]

Thanks! I will fix this as soon as possible. The process is a bit cumbersome, as I have to commit, rebase and tag again to keep the book in sync with the repository, so I can't just merge this PR. I will keep it open until I'm done with the process. Thanks again!