ci(python): run lint / unit tests / docs as GH actions

[parthea]

I tested the changes using the following commands after cloning synthtool

git checkout add-lint-unittest-docs-checks-as-github-actions
docker build -f docker/owlbot/python/Dockerfile -t movetoghactions .

Once the docker image was built, I ran the following commands in the root of python-access-approval.

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

This should not impact the existing kokoro or docs pre-submit jobs as we are adding github action checks in parallel. Once this is merged, we can disable the kokoro/docs on a per repo basis.

[parthea]

Wait for EOD to merge to avoid hitting kokoro github quota