chore: add 3.9 to noxfile template (#909)

Since the python-docs-samples noxfile-template doesn't sync with this, I wanted to make sure the noxfile template matched the most recent change [here](https://github.com/GoogleCloudPlatform/python-docs-samples/pull/4968/files)

cc @tmatsuo

Author: leahecole

synthtool/gcp/templates/python_samples/noxfile.py.j2

@@ -85,7 +85,7 @@ def get_pytest_env_vars() -> Dict[str, str]:
 
 # DO NOT EDIT - automatically generated.
 # All versions used to tested samples.
-ALL_VERSIONS = ["2.7", "3.6", "3.7", "3.8"]
+ALL_VERSIONS = ["2.7", "3.6", "3.7", "3.8", "3.9"]
 
 # Any default versions that should be ignored.
 IGNORED_VERSIONS = TEST_CONFIG['ignored_versions']