Bump pytest in /{{cookiecutter.app_name}}/requirements

Bumps [pytest](https://github.com/pytest-dev/pytest) from 5.3.5 to 5.4.0.
- [Release notes](https://github.com/pytest-dev/pytest/releases)
- [Changelog](https://github.com/pytest-dev/pytest/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/pytest-dev/pytest/compare/5.3.5...5.4.0

)

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