Update flask-sqlalchemy to 2.3.1

[pyup-bot]

There's a new version of Flask-SQLAlchemy available.
You are currently using 2.2. 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

2.3.0

---

Released on September 28, 2017

• Multiple bugs with __tablename__ generation are fixed. Names will be
  generated for models that define a primary key, but not for single-table
  inheritance subclasses. Names will not override a declared_attr.
  PrimaryKeyConstraint is detected. (541_)
• Passing an existing declarative_base() as model_class to
  SQLAlchemy.__init__ will use this as the base class instead of creating
  one. This allows customizing the metaclass used to construct the base.
  (546_)
• The undocumented DeclarativeMeta internals that the extension uses for
  binds and table name generation have been refactored to work as mixins.
  Documentation is added about how to create a custom metaclass that does not
  do table name generation. (546_)
• Model and metaclass code has been moved to a new models module.
  _BoundDeclarativeMeta is renamed to DefaultMeta; the old name will be
  removed in 3.0. (546_)
• Models have a default repr that shows the model name and primary key.
  (530_)
• Fixed a bug where using init_app would cause connectors to always use the
  current_app rather than the app they were created for. This caused issues
  when multiple apps were registered with the extension. (547_)

.. _530: pallets-eco/flask-sqlalchemy#530
.. _541: pallets-eco/flask-sqlalchemy#541
.. _546: pallets-eco/flask-sqlalchemy#546
.. _547: pallets-eco/flask-sqlalchemy#547

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

Happy merging! 🤖