Format code using black

[ianhi]

closes: #728

1. Formatted existing code black .
2. added black . --check to github actions
3. Saved formatting commit SHA in a file following https://github.com/psf/black/tree/20.8b1#migrating-your-code-style-without-ruining-git-blame
4. added pre-commit to setup.py and updated install instructions (https://github.com/psf/black/tree/20.8b1#version-control-integration)

@mlucool @fcollonval

[ianhi]

Tests fail because black is not available on python 3.5. This can be worked around with some if statements but it seems to get a little bit ugly as github actions don't support line level if statements. Given that python 3.5 is at the end of life in 9 days (september 13) I propose dropping it from the tests. Thoughts?

[fcollonval]

Thanks @ianhi

I agree with you. We can drop testing against 3.5 and I went ahead by adding 3.9rc.

I reorganize a bit the CI script so only jupyterlab needs to be explicitly installed before the test step (to get jlpm).

[fcollonval]

Very nice

[fcollonval]

@meeseeksdev backport to 0.11.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

$ git checkout 0.11.x
$ git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 a4d5970c04fd46d77611e548f4f9539e6b702e1a

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am 'Backport PR #763: Format code using black'

4. Push to a named branch :

git push YOURFORK 0.11.x:auto-backport-of-pr-763-on-0.11.x

5. Create a PR against branch 0.11.x, I would have named this PR:

"Backport PR #763 on branch 0.11.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to suggest an improvement.