Use Ruff for linting and formatting

[breakid]

Ruff is "[a]n extremely fast Python linter and code formatter, written in Rust." It supports, and consolidates, checks from a wide variety of other tools (e.g., flake8, isort, pydocstyle, pylint, etc.).

• Added Ruff config to pyproject.toml
• Fixed linting errors

Opinionated Changes

• Increased the required Python version 3.9, since everything older is past end-of-life
  • Ruff did not support Python 3.4
• Increased the max line-length to 100 characters to improve readability on modern monitors
  • I typically use 120 characters but choose a more moderate 100 as a compromise
  • This can be easily reverted, if desired

[breakid]

I can't figure out why the pypy tests are failing. What's even weirder is that they fail on the master branch for me, despite clearly passing in the last pull request merged.

[mikf]

Thanks, but I have no interest in using Ruff or anything other than flake8 for linting, especially if it would mean removing support for Python 3.8 and lower as well as reformatting 500+ files and having to re-setup all my (local) tool chains.

Maybe in the far future, but for now flake8 is more than good enough.

I can't figure out why the pypy tests are failing. What's even weirder is that they fail on the master branch for me, despite clearly passing in the last pull request merged.

This error was unrelated to your changes an got "fixed" in bced143

[breakid]

Fair enough. I have the Ruff extension installed in my IDE, and it automatically started making changes, so I figured I'd make the suggestion.

Thank you for letting me know about the pypy issue.