Replace flake8 + black + isort with `ruff`

[subhashb]

Task

Remove flake8, autoflake, black, and isort and replace them with ruff. Also, remove tox completely and depend on Github actions for multi-env testing.

Justification

ruff is a single tool and acts as a drop-in replacement for all these tools. It's config is in one place, in pyproject.toml. And it's blazing fast!