blacken samples using templated noxfile

[tswast]

This repository has three historical samples locations (see #790 for the issue to migrate them all to the most recent layout, supported by templates), but this issue only refers to the samples in (3). The samples in (1, 2) should remain blackened by the existing mechanism:

1. docs/snippets.py
2. samples/*.py + samples/tests/*.py
3. samples/snippets/*.py, samples/geography/*.py, samples/magics/*.py, and possibly more in the future, see: Clean-up samples/snippets/requirements.txt #1352.

For this issue, I anticipate the following updates:

1. Update the blacken session in noxfile.py to only blacken the samples in legacy directories. That is

   python-bigquery/noxfile.py

   Line 28 in 34a3f5c

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

   should be updated to BLACK_PATHS = ("docs", "google", "samples/*.py", "samples/tests/*.py", "tests", "noxfile.py", "setup.py") or similar.
2. Update owlbot config to make sure the code samples in samples/snippets|geography|etc stay blackened. We still want to run nox -s blacken in the root directory but then we also want to run nox -s blacken in each samples directory that has a noxfile.py. See the implementation in python-bigquery-datatransfer for reference: https://github.com/googleapis/python-bigquery-datatransfer/blob/822223fc02b1e2ff6f52b834c51f4bf46924e2e8/owlbot.py#L60-L61

[tswast]

Turns out these are already blackened, but they aren't using the blacken session from the templated noxfile.

[tswast]

Unblocked by #785