SA20: Resolve all deprecation warnings using `SQLALCHEMY_WARN_20=1`

[amotl]

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When running the test suite, it triggers a few deprecation warnings:

<stdin>:1: MovedIn20Warning: The ``declarative_base()`` function is now available as sqlalchemy.orm.declarative_base(). (deprecated since: 1.4) (Background on SQLAlchemy 2.0 at: https://sqlalche.me/e/b8d9)
<stdin>:1: RemovedIn20Warning: The ``bind`` argument to declarative_base is deprecated and will be removed in SQLAlchemy 2.0. (Background on SQLAlchemy 2.0 at: https://sqlalche.me/e/b8d9)

I was following the steps described here, but on the following connection.execute I get a crate.client.exceptions.ProgrammingError: RelationUnknown[Relation 'locations' unknown]. Is there a step missing to run the table creation? And how does it later return a row count of 13, where are these rows coming from?

References

• Refactor certain doctests towards either reStructuredText documentation or Python tests crate-python#464
• SA20: Evaluate compatibility with SQLAlchemy 2.0 #109

Originally posted by @hammerhead in crate/crate-python#464 (comment)

[amotl]

I think I had already made a start on this at a single spot when looking at SA20 compatibility. See db3831a05480. However, let's dedicate a patch to getting rid of deprecation warnings only, without making further changes.

[amotl]

Resolved with crate/crate-python#485.