testing blacken as part of owlbot processing using 3.8

Author: chalmerlowe

noxfile.py

@@ -103,7 +103,7 @@ def lint(session):
     session.run("flake8", "db_dtypes", "tests")
 
 
-@nox.session(python=DEFAULT_PYTHON_VERSION)
+@nox.session(python="3.8")
 def blacken(session):
     """Run black. Format code to uniform standard."""
     session.install(BLACK_VERSION)