Bump ethereum_ssz from 0.8.3 to 0.9.0

Bumps [ethereum_ssz](https://github.com/sigp/ethereum_ssz) from 0.8.3 to 0.9.0.
- [Release notes](https://github.com/sigp/ethereum_ssz/releases)
- [Commits](sigp/ethereum_ssz@v0.8.3...v0.9.0)

---
updated-dependencies:
- dependency-name: ethereum_ssz
  dependency-version: 0.9.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Author: dependabot[bot]