Add code style support for Python

[grayside]

Proposal

Follow-up #6 with code style support for Python. Per @dinagraves, use black of flake8.

• Add a github action that runs lint & makes code suggestions for python code.
• If we have local dev setup in place, add the same lint configuration to run locally.
• For local dev, document the tools to use to align with our python style.

Problems this will solve

• Keep consistent code style
• Reduce human attention during reviews to address common readability challenges.

[dinagraves]

We use the following flake8 args in python-docs-samples. https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/noxfile-template.py#L131

I don't know if there is a way to make flake8 automatically fix the style and import issues it finds. If we want to do that, we might want to use black for the linting and isort with autoflake for the imports.

[ace-n]

(Also: we may be able to use black for local linting and perform the rest via GitHub Actions.)