Introduce a code formatter into the tahoe

[fenn-cs]

It would be nice to make code formatting a part of tahoe's linting process.

Our options are :

• ​https://github.com/psf/black
• ​https://github.com/google/yapf

[fenn-cs]

Researched on code formatting approaches we can take for a large code base like Tahoe. Options include

• Entire codebase reformat in giant commit (Mostly bad)
• Module by module introduction
• Progressive introduction (with a pre commit hook that enforces formatting rules on new files or change files)

​https://blog.readme.com/adopting-coding-standards-within-a-large-codebase/

[meejah]

A good branch to experiment with for how to bring an "old" branch up-to-date after a black-ening is: ​https://github.com/meejah/tahoe-lafs/tree/2916.grid-manager-proposal.5

(There are others with .4, etc that are older). The above probably still has "legitimate" conflicts that I'll need to address as master hasn't been merged into it in a while.