Bump protobuf from 3.8.0 to 3.15.0 in /py/kubeflow/tf_operator

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.8.0 to 3.15.0.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.8.0...v3.15.0)

---
updated-dependencies:
- dependency-name: protobuf
  dependency-type: direct:production
...

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