Restore usage of flake8 and add to CI

[danyeaw]

In #315, I accidently removed the flake8 configuration that was in the setup.cfg. This PR restores it and makes some additional improvements:

• Fixes issue detected by flake8
• Add a pre-commit hook to make it easier to run on each commit
• Add additional pre-commit hooks to fix trailing whitespace and end of files
• Add a step to CI to lint the changes

It would also be great to eventually move to using Ruff like jupyter/notebook, but that would be a much larger cleanup effort.

[RRosio]

Thank you @danyeaw!

I was thinking that it might be helpful to have the pre-commit information in the CONTRIBUTING.md file? It would be a nice reminder. Maybe something like:

---

To apply code formatting before committing changes, install pre-commit and the pre-commit git hooks. The hooks are defined in .pre-commit-config.yaml.

pip install pre-commit
pre-commit install
pre-commit run

---

I can merge this in and follow up on the documentation, or if you'd like to included here as well? Please let me know your thoughts!

[danyeaw]

Hi @RRosio, thanks so much for the feedback! Clear contributor instructions are really important and this is a great idea to improve this PR. I updated the CONTRIBUTING.rst file.

I was also getting a CI failure because v1 of check-release was using a really old version of actions/cache, so I bumped it to v2 as well.

[RRosio]

Thank you @danyeaw!