[CI] Migrate from YAPF to Black

[bveeramani]

Overview

The Ray Team has decided to format the Ray Python code with Black. This means that going forward, changes to Python code must adhere to the Black code style.

We'll be updating ./scripts/format.sh, so if you're a contributor, your workflow shouldn't change.

Motivation

We're switching to Black because:

• Code formatted with Black is more readable than code formatted with YAPF.
• Black is faster than YAPF.
• Black is the most popular Python formatting tool.

For more information about the motivation behind this change, read the full proposal.

Implementation Plan

We're planning on rolling out the changes in three phases.

Phase One:

• [CI] Update format.sh to support the Black code style #21312
• [CI] [RLlib] flake8 raises E731 after code is formatted with Black #21315
• [CI] [RLlib] Prevent Black from erroring while formatting multi_agent_env.py #21314
• [CI] [Core] Prevent Black from erroring while formatting test_resource_demand_scheduler.py #21313

Phase Two:

• [CI] Format Python code with Black #21316
• Comment on open pull requests and tell contributors how to update their code.

Phase Three:

• [Documentation] Change code style from Google to Black #21317
• [CI] Remove or replace YAPF disables #21318
• [CI] Remove YAPF from format.sh #21319

Timeline

Phase	Due Date	Status
I	1/1	✅ Done
II	1/15 1/29	✅ Done
III	1/31	✅ Done