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