Bump black from 23.3.0 to 23.7.0 in /backend/paaster

Bumps [black](https://github.com/psf/black) from 23.3.0 to 23.7.0.
- [Release notes](https://github.com/psf/black/releases)
- [Changelog](https://github.com/psf/black/blob/main/CHANGES.md)
- [Commits](psf/black@23.3.0...23.7.0)

---
updated-dependencies:
- dependency-name: black
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>