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

[parthea]

Many python repositories have a separate noxfile.py file in the samples directory. This PR modifies owlbot_main() for python to run the blacken nox session on all directories that contain a noxfile.py rather than only the root of the repository.

I tested the changes using the following commands after cloning synthtool

git checkout run-blacken-session-for-all-dirs
docker build -f docker/owlbot/python/Dockerfile -t runblackenonsamples . 

Once the docker image was built, I ran the following commands in the root of python-apigee-connect which has no samples and python-secret-manager which has samples. I've confirmed that the blacken session runs on all directories that have noxfile.py.

docker run --user $(id -u):$(id -g) --rm -v $(pwd):/repo -w /repo runblackenonsamples

[parthea]

@busunkim96, Please could you review/approve?

[busunkim96]

I think this will fail in google-auth because it has a noxfile.py with no blacken. Is there a way to opt out for that repo?

https://github.com/googleapis/google-auth-library-python/blob/main/system_tests/noxfile.py

[parthea]

@busunkim96 Good find! I think it won't be an issue as the change is in owlbot_main(). This will only affect repos that don't have an owlbot.py. See
https://github.com/googleapis/synthtool/blob/master/synthtool/languages/python.py#L153