make all run files py3 only (GoogleCloudPlatform#3024)

Co-authored-by: Bu Sun Kim <[email protected]>

Author: leahecole

noxfile-template.py

@@ -154,7 +154,7 @@ def _setup_appengine_sdk(session):
             "bigquery/pandas-gbq-migration"
         )
         or str(Path(sample).absolute().relative_to(REPO_ROOT)).startswith(
-            "run/system-package"
+            "run/"
         )
     )
 ]