Bump django-upgrade from 1.19.0 to 1.20.0 in /requirements

Bumps [django-upgrade](https://github.com/adamchainz/django-upgrade) from 1.19.0 to 1.20.0.
- [Changelog](https://github.com/adamchainz/django-upgrade/blob/main/CHANGELOG.rst)
- [Commits](adamchainz/django-upgrade@1.19.0...1.20.0)

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

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