Add black and flake8 to pre-commit, and run it in GitHub Actions

[mhsmith]

The easiest way to make sure the code stays compliant with black and flake8 is to give the repository a pre-commit configuration, so the checks will be run before every commit.

In case anyone forgets to enable pre-commit locally, it should also be run by GitHub Actions in each PR.

[mhsmith]

At the same time, I think it would also be a good idea to switch to black's default line length of 88 characters.

[ntoll]

I grew up using vi at 80 chars in a terminal. 88 characters would require me to have a lobotomy. :wink:

(But yeah, this seems to be the way to go...)

[mhsmith]

Explanation from the black developers here.