Bump flask-migrate in /{{cookiecutter.app_name}}/requirements

Bumps [flask-migrate](https://github.com/miguelgrinberg/flask-migrate) from 2.5.2 to 2.5.3.
- [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/v2.5.2...v2.5.3

)

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