Adopt black as code formatter

[sfdye]

This PR proposes to use black as the code formatter.

• Apply black to whole code base
• Apply isort to whole code base
• Make flake8 compatible with black
• Update docs
• Update PR template
• Enforce black on CI
• Enforce isort on CI
• Add pre-commit
• Add .ignore-rev-files

[sfdye]

@s-t-e-v-e-n-k Let me know if you have any strong opinion on this PR

[s-t-e-v-e-n-k]

@sfdye My strong opinion is this all looks amazing! If you're happy with it, I say let's land it and cut a release.

[TravisBuddy]

Hey @sfdye,
Your changes look good to me!

View build log

TravisBuddy Request Identifier: 491877c0-1b2d-11ea-b9ce-6f43500ed087

[sfdye]

@s-t-e-v-e-n-k Thanks, merged into master. In the meantime, please hold off merging any open PR into master until I enforce black and isort on CI.

[s-t-e-v-e-n-k]

@sfdye Noted. I have a large branch to drop 2.7 support locally, but we need a release first ,so drop me a line when you're ready for me to pull the trigger for that (there's a breaking change we need to document carefully as well as stating this will be the last release to support 2.7)