feat!: Refactor most of the files for tests to pass on sqlalchemy2.

[bvanelli]

Add supports to SQLAlchemy 2.0, at least where the tests are concerned:

• Migrate all references of relation to relationship instead
• Use session.add() for all objects that were not added to the session. Previously, the objects were added immediately after an instance was created
• Added engine.dispose on the book close to make sure the database can be deleted.
• Modified all tests to use the open_book() as a contextmanager, to prevent the deleting of the database files to raise an error of being opened in another process
• Replace references to engine.execute by conn.execute instead.
• Bumped some versions on requirements to make it installable for me (windows 10, python 3.10), but I believe some other versions can be bumped too.

Closes #218

TODOS:

• Correct examples/ files, as they also need to use the new format from sqlalchemy
• Change documentation where is required
• Fix failing test def test_create_with_FK(self): that I could not fix