Add support for SQLAlchemy 2.1

[ollz272]

Summary

import sqlalchemy_utils fails on SQLAlchemy 2.1:

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

SQLAlchemy 2.1 made several attribute internals private. This PR:

• Imports _ScalarAttributeImpl / _register_attribute on 2.1+, falling back to ScalarAttributeImpl / register_attribute on 2.0 (used by generic_relationship).
• Makes the version check in GenericAttributeImpl tolerate pre-release version strings (e.g. 2.1.0b1), which previously raised ValueError.
• Pins the test PostgreSQL DSN to postgresql+psycopg2://. SQLAlchemy 2.1 changed the default postgresql:// driver to psycopg (v3), which CompositeType doesn't support; this keeps the tests using the same driver as before.

Testing

Full suite run locally against PostgreSQL and MySQL on both SQLAlchemy 2.1.0 and 2.0.54: 2690 passed on each, with identical results. The one failure on both is the psycopg2cffi test, because that driver isn't installed locally. MSSQL was not tested locally.

Notes

• CompositeType still only supports psycopg2. On SQLAlchemy 2.1, users with a plain postgresql:// URL now get psycopg3 and a TypeError about cursor_factory. They need postgresql+psycopg2:// for now. psycopg3 support could come in a follow-up.
• tox installs the latest SQLAlchemy, so CI will now test 2.1 only. It may be worth adding a 2.0 job to the matrix.

[kurtmckee]

sqlalchemy is explicitly discouraging this usage. Changing sqlalchemy-utils to support this usage doesn't align with what upstream is explicitly trying to accomplish.

I'm open to a solution that doesn't rely on sqlalchemy internals like this, though!