Finish adoption / enforcement of Black code formatter

[bcantoni]

In #835, Tyler formatted the whole tree with Black and removed pycodestyle from the GitHub workflow. At the time he mentioned that it was fully implemented yet in the workflow.

In parallel I had added steps for using a pre-commit hook that included pycodestyle. I'm assuming not many people are using that because the current source tree doesn't pass either style at this point. :)

Running black --check --line-length 120 tableauserverclient samples test on the development branch shows 71 files which would be reformatted.

I'm inclined to finish the work of implementing black:

1. Swapping black for pycodestyle in the setup dependencies
2. Edit the pre-commit hook example in docs
3. Add a short explanation of black in the developer guide
4. Add black to the workflow
5. Do another big commit to make everything compliant with black again

[bcantoni]

It looks like #935 covers everything except for 2&3 - I'll create a docs PR for those.