[CI] Update `format.sh` to support the Black code style

[bveeramani]

Summary

Update format.sh so that developers can format code in the Black code style, but continue to use YAPF by default.

Motivation

The pull request to Blacken the code will modify over 1,500 files. To ensure that the changes to format.sh are easily reviewable, we'll make the changes in a separate PR.

Changes

Here are all of the specific changes we'll need to make to implement this functionality:

• Add black==21.12b0 to python/requirements_linters.txt
• Add black==21.12b0 to python/ray/tune/requirements-dev.txt
• Add black==21.12b0 to .gitpod/Dockerfile
• Add the ability to format code with Black using format.sh, but guard the changes behind a flag BLACK_FORMATTER_ENABLED. When we format the code with Black, we'll set the flag to true and make Black the default formatter.
• Update .flake8 to make flake8 compatible with Black. To learn more about the changes we'll need to make, see the relevant section of the Black documentation. Note that none of these changes will cause issues with our current setup.