[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.

setup.py

@@ -346,6 +346,7 @@ def do_setup():
             'jinja2>=2.10.1, <2.11.0',
             'lazy_object_proxy~=1.3',
             'markdown>=2.5.2, <3.0',
+            'marshmallow-sqlalchemy>=0.16.1, <0.19.0',
             'pandas>=0.17.1, <1.0.0',
             'pendulum==1.4.4',
             'psutil>=4.2.0, <6.0.0',