Use the new-style sqlalchemy DeclarativeBase

[chrisburr]

Actually we should use https://docs.sqlalchemy.org/en/20/changelog/whatsnew_20.html#migrating-an-existing-mapping so we can type this as Declarative base.

For another PR.

Originally posted by @chrisburr in #608 (comment)

We can then drop the use of Any in the _search endpoint.

[aldbr]

Here we just want to follow the documentation above.

• probably just a matter of converting from one way to another.
• Example from copilot (may be not so reliable):

Before:

from sqlalchemy.orm import declarative_base
Base = declarative_base()

# Type hints using Any
async def _search(self, table: Any, ...):

After:

from sqlalchemy.orm import DeclarativeBase

class Base(DeclarativeBase):
    pass

# Proper type hints
async def _search(self, table: type[DeclarativeBase], ...):

[ryuwd]

Example of how I did it for the Bookkeeping schema

https://gitlab.cern.ch/lhcb-dirac/lhcbdiracx/-/blob/master/lhcbdiracx-db/src/lhcbdiracx/db/sql/bookkeeping/schema.py?ref_type=heads