Bump flask-migrate from 2.3.1 to 2.4.0 in /{{cookiecutter.app_name}}

Bumps [flask-migrate](https://github.com/miguelgrinberg/flask-migrate) from 2.3.1 to 2.4.0.
- [Release notes](https://github.com/miguelgrinberg/flask-migrate/releases)
- [Changelog](https://github.com/miguelgrinberg/Flask-Migrate/blob/master/CHANGELOG.md)
- [Commits](https://github.com/miguelgrinberg/flask-migrate/compare/v2.3.1...v2.4.0)

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

{{cookiecutter.app_name}}/Pipfile

@@ -18,7 +18,7 @@ psycopg2 = "==2.7.7"
 SQLAlchemy = "==1.2.18"
 
 # Migrations
-Flask-Migrate = "==2.3.1"
+Flask-Migrate = "==2.4.0"
 
 # Forms
 Flask-WTF = "==0.14.2"