chore(bigquery): use 3.6 for blacken session (#10012)

* [CHANGE ME] Re-generated bigquery to pick up changes in the API or client library generator.

* chore: use 3.6 for blacken session

* chore: blacken

* chore: add comment about why 3.6 is used

* chore: revert changes to metadata

Co-authored-by: Bu Sun Kim <8822365+busunkim96@users.noreply.github.com>

bigquery/noxfile.py

@@ -175,10 +175,14 @@ def lint_setup_py(session):
     session.run("python", "setup.py", "check", "--restructuredtext", "--strict")
 
 
-@nox.session(python="3.7")
+@nox.session(python="3.6")
 def blacken(session):
     """Run black.
     Format code to uniform standard.
+
+    This currently uses Python 3.6 due to the automated Kokoro run of synthtool.
+    That run uses an image that doesn't have 3.6 installed. Before updating this
+    check the state of the `gcp_ubuntu_config` we use for that Kokoro run.
     """
     session.install("black")
     session.run("black", *BLACK_PATHS)