fix #638 updates for SQLAlchemy 2

[moellep]

• wrap raw SQL in text() for SA 2 compatibility
• use execution_options(autobegin=False) for explicit transactions
• remove <2 cap on SQLAlchemy dependency

[moellep]

This needs more thought:
There should be a better way to access the row as a dictionary than going through ._mapping.
Clients of the API shouldn't be required to use ._mapping.
Also, sql_db methods which return a Row, like select_one() may want to return a PKDict directly.