Use 'python-3.6' for 'blacken' run. (#7064)

Closes #7063.

Author: tseaver

noxfile.py

@@ -52,7 +52,7 @@ def lint(session):
     session.run("flake8", "google", "tests")
 
 
-@nox.session(python="3.7")
+@nox.session(python="3.6")
 def blacken(session):
     """Run black.