Bump google-protobuf from 3.14.0 to 3.15.6 (#35)

Bumps [google-protobuf](https://github.com/protocolbuffers/protobuf) from 3.14.0 to 3.15.6.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.14.0...v3.15.6)

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