Add Black beautifier for Python

[arl-o]

What does this implement/fix? Explain your changes.

Adds support for the Black python beautifier.

Does this close any currently open issues?

Closes #2144

Any other comments?

Currently does not support line length option, since Black uses a different default from autopep8 and yapf which I did not want to overwrite. (Is there a supported way to set this on an executable-basis?)

I also did not see how to set up test examples for specific beautifiers, instead of specific languages. Is there a way to do this?

Checklist

• Merged with latest master branch
• Regenerate documentation with npm run docs
• Add change details to CHANGELOG.md under "Next" section
• Travis CI passes (Mac support)
• Added examples for testing to examples/ directory
• AppVeyor passes (Windows support)

[arl-o]

CI checks are failing because black requires python 3.6.0+ to run.

Should I try updating the .ymls to use pip3?

[stevenzeck]

I'm trying to remember if you can include this in .travis.yml for non-Python builds, I think you can so put this underneath language: generic

python:
  - "3.6"

For appveyor.yml, change the two C:\Python27 to C:\Python36 on this line: https://github.com/arl-o/atom-beautify/blob/master/appveyor.yml#L13.

If pip3 is necessary, change that as well.

[arl-o]

I'm running some CI tests on a new branch, will update here when that's working.

[stevenzeck]

Here's what we use for beautifier-black: https://github.com/Unibeautify/beautifier-black/blob/master/.travis.yml

[arl-o]

Arg. The above works for linux to install black, which then fails afterward because fuse, react, and various language packages, fail to load.

edit: I really can't figure out how to get pip3 working. I've tried about 2 dozen configurations, and now I'm getting errors in travis on totally unrelated lines like No available formula with the name "composer" .

[arl-o]

Reverted to a version that doesn't install Black for travisCI checks. No idea why AppVeyor is now failing on apm install --packages-file atom-packages.txt, since nothing was changed from the main branch that should affect it.

[stevenzeck]

Yea I can't install language-marko locally either. Not sure how Travis was able to pass.

[arl-o]

Looks like appveyor was also failing on the master branch for the same reason.

Also I accidentally hit close. Many mistakes.

[stevenzeck]

Thanks for contributing!