Support sqlalchemy>=2

[mgorny]

When sqlalchemy==2.0.19 is installed, the relevant tests are skipped. When I try to import the relevant module, I'm getting the following ImportError:

>>> import sphinxcontrib.websupport.storage.sqlalchemy_db
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.11/site-packages/sphinxcontrib/websupport/storage/sqlalchemy_db.py", line 12, in <module>
    from sqlalchemy.orm import relation, sessionmaker, aliased
ImportError: cannot import name 'relation' from 'sqlalchemy.orm' (/usr/lib/python3.11/site-packages/sqlalchemy/orm/__init__.py)