use black to format python code

[li-wu]

1. Introduce black as code format tool;

• Format all python code using black
make format-all
• Format changed or newly added python codes using black before commit:
  make format

And you can still use flake8 to lint the code:

• Lint all python code using flake8
make lint-all
• Format changed or newly added python codes using black before commit:
  make lint

2. All changed python files are formatted using black

3. Fix some issues reported by flake8

• unused imports
• unused variables
• ambiguous variables

[li-wu]

Use black instead of yapf here.

[li-wu]

This is the configuration file for black.