[Gardening] Update all Python scripting to conform to the style guidelines enforced by the black code-formatting tool.

[Rostepher]

The black tool is the Python Software Foundation's blessed formatting tool in the same spirit as gofmt. It's opinionated, but keeps all code consistent automatically. This change is completely non-functional and merely adds a proper flake8 config file and runs the black tool over the build-script.py and lit_tests/lit.cfg files.

Note: black is MIT licensed.

[Rostepher]

@swift-ci please test

[ahoppen]

Looking good.

As a general direction: Are you planning to add CI testing to enforce the code style?

[Rostepher]

@ahoppen I would love to add that in the future. We could enforce flake8 and black on all the Python sources. But for now this is more a manual transition.