fix SQLAlchemy deprecated execute param in test

[zermelo-wisen]

Fixes #279.

• Fixed deprecated usages in tests.
  -- Passing a string to Conneciton.execute() is deprecated in SQLAlchemy 1.4 and removed in 2.0: https://docs.sqlalchemy.org/en/14/core/connections.html#sqlalchemy.engine.Connection.execute
  -- Changed the tests with warnings issuing a DML with Connection.execute() to be explicit about commit behavior since auto-commit behavior is changed in 2.0: https://docs.sqlalchemy.org/en/20/changelog/migration_20.html
• Updated tox config to test with both 1.4 and 2.0 versions of SQLAlchemy.

[apotterri]

SQLAlchemy v1.4 is still getting updates, so we need to continue testing against it.

Can you update the tox config to include an environment that does that, in the same way we test old versions of Django and Flask, please?

[zermelo-wisen]

SQLAlchemy v1.4 is still getting updates, so we need to continue testing against it.

Can you update the tox config to include an environment that does that, in the same way we test old versions of Django and Flask, please?

Done.