Minor update based on changes in SQLAlchemy 2.0

[mitchnegus]

Apologies for not submitting an issue first, but was hoping this would be minor enough to not need one.

Looks like the autoload option to the Table constructor was deprecated in the transition to SQLAlchemy 2.0 and the recommended usage is autoload_with=engine. I included that new usage alongside the old in the README (mostly to help any new-ish SQLAlchemy users like myself from hunting through the Table signature trying to find the non-existent autoload parameter 😅).