Bump h2 from 4.1.0 to 4.3.0 in /example/backend

Bumps [h2](https://github.com/python-hyper/h2) from 4.1.0 to 4.3.0.
- [Changelog](https://github.com/python-hyper/h2/blob/master/CHANGELOG.rst)
- [Commits](python-hyper/h2@v4.1.0...v4.3.0)

---
updated-dependencies:
- dependency-name: h2
  dependency-version: 4.3.0
  dependency-type: indirect
...

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Author: dependabot[bot]