deps: add protobuf dependency

[anistark]

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Fixes #305

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[anistark]

@googlebot I signed it!

[tswast]

Thanks for the contribution. Even though this should be covered by our transitive dependencies, I think it's worth calling out in this package as well.

Could you also pin protobuf here: https://github.com/googleapis/python-bigquery/blob/master/testing/constraints-3.6.txt ? That's needed to test that the minimum version is correct.

[tswast]

Re: conventionalcommits.org failure

The allowed prefixes are defined here: https://github.com/googleapis/release-please/blob/master/src/releasers/python.ts This commit should be prefixed with deps:

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[HemangChothani]

@googlebot I consent