Made pre-commit the only way to run Flake8 and isort.

[adamchainz]

Prior to this commit there were four(-ish) ways sitting around for running the tools:

• The doc recommendations to install and run the tools yourself.
• Tox environments.
• The GitHub Actions Linters run.
• pre-commit.

This is a lot of overhead, muddies the path for contributors, and it has the risk of new tool versions being used inconsistently. pre-commit is the most mature and capable solution, so I’d like to recommend using it exclusively.

For running the hooks on GitHub, I’d like to try pre-commit.ci. It's by the same author, free for open source, hella fast, and pushes back fixes. I've used it successfully on my repos for a long time now. The second commit configures it to use Djangoey commit messages, and avoid spamming version updates (personally I'm happy with weekly updates on my repos).

It will require a Fellow to register the repo with pre-commit.ci and give it access on GitHub, as they have commit access.