Update flask_sqlalchemy to 2.4.0

[pyup-bot]

This PR updates flask_sqlalchemy from 2.3.2 to 2.4.0.

Changelog

2.3.3

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Fix "AttributeError: 'NoneType' object has no attribute 'info'", when using polymorphic models. (`651`_)

.. _651: https://github.com/mitsuhiko/flask-sqlalchemy/pull/651

Links

• PyPI: https://pypi.org/project/flask-sqlalchemy
• Changelog: https://pyup.io/changelogs/flask-sqlalchemy/
• Repo: https://github.com/pallets/flask-sqlalchemy
• Docs: https://pythonhosted.org/Flask-SQLAlchemy/