[RFC] Adopting black Python formatter.

[trivialfis]

The black Python formatter supports automated code formatting. It's widely used on various Python projects like dask and optuna and I really like its output. It allows me to write code without wrangling with formats, which is becoming more and more difficult with Python typing.

However currently xgboost's formatting style is quite different from the default in black so black will generate a huge diff. It's "uncompromising" so we can't avoid it.

[trivialfis]

Another good alternative is yapf, which is mush lesser intrusive.

[trivialfis]

Closing for now. Tried both formatters, I think this has to be an incremental effort, otherwise the diff is just too huge.