catch AttributeError instead of TypeError

[davidism]

In Flask-SQLAlchemy 3.1, the deprecated get_engine method will be removed, after raising a DeprecationWarning in 3.0. 7cb4236 caught TypeError, but a method that doesn't exist raises an AttributeError.

[davidism]

I've kept get_engine for one more version in Flask-SQLAlchemy 3.1, so it will now be removed in 3.2, meaning the current env.py will not start failing yet. The old bind argument is accepted so users with the current env.py see the deprecation message. The message mentions Flask-Migrate (and Alembic) and tells users to update env.py, so hopefully this will make the transition a little smoother.