Update SQLAlchemy example to use txn retries

Estimated scope of work:

- Update the code to use the `run_transaction` API as described in the *Handling restarted transactions* section of the [Building an application with CockroachDB and SQLAlchemy](https://www.cockroachlabs.com/blog/building-application-cockroachdb-sqlalchemy-2/) blog post.
- Update tests, other things (?) as needed.