Bump flask-sqlalchemy from 2.3.2 to 2.4.0

Bumps [flask-sqlalchemy](https://github.com/pallets/flask-sqlalchemy) from 2.3.2 to 2.4.0.
- [Release notes](https://github.com/pallets/flask-sqlalchemy/releases)
- [Changelog](https://github.com/pallets/flask-sqlalchemy/blob/master/CHANGES.rst)
- [Commits](https://github.com/pallets/flask-sqlalchemy/compare/2.3.2...2.4.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

setup.py

@@ -33,7 +33,7 @@ setup(
     packages=find_packages(exclude=['tests', 'tests.*']),
     install_requires=[
         'Flask==1.0.2',
-        'flask-sqlalchemy==2.3.2',
+        'flask-sqlalchemy==2.4.0',
         'Flask-Migrate==2.4.0',
         'flask-bootstrap==3.3.7.1',
         'flask-nav==0.6',