Add support for SQLAlchemy 2.1

[ollz272]

Follow-up to #830, reworked based on the feedback there: this no longer touches any SQLAlchemy internals.

Summary

import sqlalchemy_utils fails on SQLAlchemy 2.1:

AttributeError: module 'sqlalchemy.orm.attributes' has no attribute 'ScalarAttributeImpl'

The only user of those internals was generic_relationship. Rather than importing the now-private _ScalarAttributeImpl / _register_attribute, this PR reimplements it as a hybrid_property with a custom comparator, using only public APIs:

• Lazy load via object_session() + Session.get() (replaces the private _state_session).
• Target class lookup via inspect(cls).registry.mappers (replaces registry._class_registry).
• Column → attribute resolution via Mapper.get_property_by_column().
• is_type() via Mapper.self_and_descendants (replaces _inheriting_mappers).
• The GenericAttributeImpl constructor version-sniffing hack is gone.

The public interface is unchanged: generic_relationship(discriminator, id, doc=None), column/string/hybrid discriminators, composite ids, declared_attr on abstract bases, Event(object=obj), filter_by(object=obj), Event.object == obj, !=, and is_type().

Behaviour changes

• Assignment now sets the discriminator and id attributes through normal instrumentation instead of writing directly to the instance dict. As a result, re-assigning a generic relationship on a persistent object is now actually flushed (previously the change was not tracked).
• Event.object is a hybrid rather than a MapperProperty, so it appears in mapper.all_orm_descriptors but not mapper.attrs.
• A misconfigured discriminator/id column raises ImproperlyConfigured on first use rather than at mapper configuration.

Tests

Also pins the test PostgreSQL DSN to postgresql+psycopg2://. SQLAlchemy 2.1 changed the default postgresql:// driver to psycopg (v3), which CompositeType doesn't support.

Testing

• All tests/generic_relationship tests pass on SQLAlchemy 2.0.54 and 2.1.0.
• Checked manually on both versions: reading back before any session, re-assigning on a persistent object and flushing, select().where(Event.object == obj), is_type(), and doc=.
• The rest of the suite was run against SQLite only for this revision (no PostgreSQL/MySQL/MSSQL available locally). Results were identical before and after the change on both SQLAlchemy versions.

Notes

• CompositeType still only supports psycopg2; on 2.1, a plain postgresql:// URL gets psycopg3. psycopg3 support could be a follow-up.
• tox installs the latest SQLAlchemy, so CI will test 2.1 only. It may be worth adding a 2.0 job to the matrix.

[danilogco]

#829

[ollz272]

Hi @kurtmckee

Just wondering if you could take a look at this one? The previous was rejected quite swiftly, really hoping we can get a release out that supports 2.1.