pre-commit and pre-commit-ci

[aucampia]

Add pre-commit and pre-commit-ci with the following hooks:

• black for formatting code.

For more info see docs/developers.rst.

[aucampia]

@eggplants Please have a look if you have a moment and let me know if you have any feedback.

[aucampia]

For a demo see: aucampia#33

I have enabled pre-commit CI now, but until this is merged it will always fail, it can be easily disabled from here: https://pre-commit.ci/

If we are in agreement on the settings there are the following options to actually run the hooks on the codebase:

1. We merge it and then wait for the weekly pre-commit CI auto fix - this happens 18:00 UTC Monday
2. We merge it and then make a emptyish PR and run pre-commit.ci autofix on it

Once this is done things will be fairly clean going forward, and each contributor can then either run pre-commit run by themselves or they/us can run pre-commit.ci autofix on their PR branches.

[aucampia]

Pycln is fairly conservative on what it cleans, it does actually end up leaving a bunch of unused imports, but only because they have side effects, for more info see: https://hadialqattan.github.io/pycln/#/?id=side-effects

[aucampia]

I will disable pre-commit.ci until this is merged. Can be controlled using github IAM at https://results.pre-commit.ci/ for future reference.

[aucampia]

Rebased to master branch.

I messed up the old demo, new one here: aucampia#35

[aucampia]

Will split the tox changes off this also as those will actually make development quite a bit easier.

[aucampia]

Given some feedback on other issues I'm going to scale this back to only run black to begin with, we can add other things afterwards.

[aucampia]

tox changes split into #1691

[aucampia]

Scaled this down to only do black.

[aucampia]

I changed the hook slightly to work with the version spec in black config, for more info see psf/black#2493.

Demo updated and re-run: aucampia#35

Will merge next week sometime.