Adopting black (and linter updates!)

[pradyunsg]

Follow up to #7084

There's basically 2 broad tasks here:

• Upgrade all of our linting tools to the latest versions (especially isort).
• Adopt black for formatting code, enforcing a consistent code style.

This should be done after the pip 20.2 release has been made, to minimise any potential disruption to the release planning/process.

[pradyunsg]

Self-assigned because I'd like to do this myself.

[pradyunsg]

#8902 updated the linters, so this is now the tracking issue for adopting black in pip's codebase.

#8903 is the "big fat PR" and @uranusjr + @pfmoore + @webknjaz rightfully pointed out that it's too big and we should break it up. #8906 is the first step in that direction.