add delete method on scoped_session

[burnettk]

for this sort of code:

user = UserModel.query.filter_by(username="awesomeuser").first()
db.session.delete(user)  # type: ignore, until now
db.session.commit()

not sure if it's a new issue in some version of sqlalchemy. i'm on SQLAlchemy-1.4.42 and Flask-SQLAlchemy-3.0.2.

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