WIP | Add pre-commit

[Pacu2]

Adding simple pre-commit as discussed in #660
Unfortunately, darker does not support pre-commit yet but opened a related pull request to change it in akaihola/darker#2

To enable pre-commit one needs simply:

1. python -m pip install -r requirements/dev.txt
2. pre-commit install

this PR consists of 3 commits:

1. requirements update
2. pre-commit configuration
3. fix for all trailing whitespace/incorrect end of file lines generated by the pre-commit

[SanketDG]

This looks like a great start!

Would you consider using darker given it's obscure enough?

I am still on the black bandwagon and I can still believe we can do it right with what I said in #660 (comment) albeit with a few blockers here and there.

Also it looks like pip-tools is being used and not used (there is an external requirements.txt file)

[timgl]

Thanks for doing this.

This does add another way of doing precommits, we already use husky for JS and I don't think they play nicely together. I personally also prefer how Husky just adds the formatted code to your commit, rather than failing and having to manually add the code back. The other nice thing is it 'just works', without manually having to install something.

[SanketDG]

@timgl Yeah, pre-commit does the same. it matters more what tool you are using (prettier/black that autoformats vs something else that just reports and exits)

[Pacu2]

@timgl Husky requires a package manager, and NodeJS to run hooks. This way, backend engineers would need to set up (and maintain) the frontend ecosystem.

Pre-commit, on the other side, just works. It supports a whole variety of scripts, no matter the programming language they use.

Not sure how to use backend-oriented hooks with Husky, however, pre-commit supports several frontend-oriented ones, not to mention eslint, tslint, prettier...

[timgl]

Happy either way! I do think we should make sure this PR doesn't break frontend formatting.

[timgl]

@Pacu2 Hello again, sorry this has stalled for a while. Do you still have interest in picking this up?

[Pacu2]

@timgl sure will do in 24h

[timgl]

This was done in #1043, but really appreciate the work!