Prepare Objects API release 3.0.3

[stevenbal]

• Resolve release blockers
  • https://github.com/maykinmedia/objects-api/pull/553/files
  • ✨ Remove django.contrib.sites #554
• Upgrade open-api-framework to latest version
• Check security tab and upgrade packages to fix vulnerabilities
• Check translations
• Bump API version number (if applicable)
  • Version bump
  • Regenerate API spec
  • Update READMEs with release dates + links
• Bump version number with bin/bump-my-version.sh bump <major|minor|patch>
• Update changelog

[stevenbal]

@Floris272 can you also upgrade kombu to >=5.4.0 to make sure maykinmedia/open-api-framework#128 is fixed for this component?

[Floris272]

kombu was already updated to 5.4.2

[stevenbal]

ah you're right, I was the one who did it even 🙃