[AIRFLOW-5430] Pin transitive dependency on marshmallow-sqlalchemy
[AIRFLOW-5430] Pin transitive dependency on marshmallow-sqlalchemy

marshmallow-sqlalchemy, a transitive dependency from Flask-AppBuilder,
broke py2 and py35 compatibility with its 0.19.0.

(cherry picked from commit eed0edde36e3c357af8eaf50893eeed71d378012)

owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow