refactor: blacken samples

[tswast]

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Fixes #765 🦕

[tswast]

I noticed some conflicts with pre-commit and the version of black used in samples sessions. Mailed googleapis/synthtool#1158 to address that.

[plamut]

I noticed some conflicts with pre-commit and the version of black used in samples sessions.

Aha, so that's why the lint check failed, one of the snippets was still re-formatted.

Ignoring that, the PR looks good.

[tswast]

Hmm... Turns out these were already being blackened by the root noxfile. Twice, actually, due to docs and samples included here:

python-bigquery/noxfile.py

Line 26 in c45a738

BLACK_PATHS = ("docs", "google", "samples", "tests", "noxfile.py", "setup.py")

Since we've still got a mess of files in the root samples/ dir like https://github.com/googleapis/python-bigquery/blob/master/samples/add_empty_column.py

I think it'd be best to wait on this until we do something like #203 (We'll have to do the move in two phases to avoid breaking cloud.google.com links)