chore(python): run blacken session for all directories with a noxfile (#1276)

Author: parthea

synthtool/languages/python.py

@@ -192,7 +192,9 @@ def owlbot_main() -> None:
 
         py_samples(skip_readmes=True)
 
-        s.shell.run(["nox", "-s", "blacken"], hide_output=False)
+        # run blacken session for all directories which a noxfile
+        for noxfile in Path(".").glob("**/noxfile.py"):
+            s.shell.run(["nox", "-s", "blacken"], cwd=noxfile.parent, hide_output=False)
 
 
 if __name__ == "__main__":