Bump flask-migrate from 2.5.1 to 2.5.2 in /{{cookiecutter.app_name}}

Bumps [flask-migrate](https://github.com/miguelgrinberg/flask-migrate) from 2.5.1 to 2.5.2.
- [Release notes](https://github.com/miguelgrinberg/flask-migrate/releases)
- [Changelog](https://github.com/miguelgrinberg/Flask-Migrate/blob/master/CHANGES.md)
- [Commits](https://github.com/miguelgrinberg/flask-migrate/compare/v2.5.1...v2.5.2)