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.

在此仓库中找到 SQLAlchemy 示例，并将其事务处理与博客的“Handling restarted transactions”部分进行比较。更新该示例以使用 run_transaction API，然后运行现有测试并在需要时进行调整。

由索引模型根据 Issue 内容生成。