Update flask-sqlalchemy to 2.3.1

[pyup-bot]

There's a new version of Flask-SQLAlchemy available.
You are currently using 2.3.0. I have updated it to 2.3.1

These links might come in handy: PyPI | Changelog | Repo | Docs

Changelog

2.3.1

---

Released on October 5, 2017

• If a model has a table name that matches an existing table in the metadata,
  use that table. Fixes a regression where reflected tables were not picked up
  by models. (551_)
• Raise the correct error when a model has a table name but no primary key.
  (556_)
• Fix repr on models that don't have an identity because they have not been
  flushed yet. (555_)
• Allow specifying a max_per_page limit for pagination, to avoid users
  specifying high values in the request args. (542_)
• For paginate with error_out=False, the minimum value for page is
  1 and per_page is 0. (558_)

.. _542: pallets-eco/flask-sqlalchemy#542
.. _551: pallets-eco/flask-sqlalchemy#551
.. _555: pallets-eco/flask-sqlalchemy#555
.. _556: pallets-eco/flask-sqlalchemy#556
.. _558: pallets-eco/flask-sqlalchemy#558

Got merge conflicts? Close this PR and delete the branch. I'll create a new PR for you.

Happy merging! 🤖