Adopting `pre-commit` and Auto Formatting

We are adopting auto-formatting, including `black` and `isort` on all files. cf #889

From the [Django Enhancement Proposal to use Black](https://github.com/django/deps/blob/main/final/0008-black.rst):

Reformatting will also be disruptive for open pull requests. One way to update them is to run Black on modified files, keep a copy aside, start a new branch from "main", move the modified files back into place, and commit the result.

Contributors will be asked to install `pre-commit` locally. We also have the option to use `@meeseeksdev please run pre-commit` on PRs if needed.