chore(python): Run owlbot_main() when owlbot.py doesn't exist

[parthea]

This PR will allow us to remove owlbot.py from python repositories that don't have repository specific owlbot.py customizations.

I tested the changes using the following commands after cloning synthtool

git checkout add-owlbot-main-for-python
docker build -t testowlbotmain -f docker/owlbot/python/Dockerfile .

Once the docker image was built, I ran the following command in the root of python-iap which has no customimzations and python-deploy which has minor customizations. Prior to running these commands, I deleted owlbot.py locally.

docker pull gcr.io/cloud-devrel-public-resources/owlbot-cli:latest
docker run --rm --user $(id -u):$(id -g) -v $(pwd):/repo -w /repo gcr.io/cloud-devrel-public-resources/owlbot-cli:latest copy-code
docker run --user $(id -u):$(id -g) --rm -v $(pwd):/repo -w /repo testowlbotmain