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.

Locate the SQLAlchemy example in this repository and compare its transaction handling with the blog's “Handling restarted transactions” section. Update the example to use the run_transaction API, then run the existing tests and adjust them as needed.

Written by the indexing model from the issue text.