Update flask-sqlalchemy to 2.3.0

[pyup-bot]

There's a new version of Flask-SQLAlchemy available.
You are currently using 2.2. I have updated it to 2.3.0

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

Changelog

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! 🤖

[pyup-bot]

Closing this in favor of #17