Summary:
Enable `black` + `isort` (via `pyfmt`) i.e. `BLACK` fbsource linter.

NOTE: the `BLACK` fbsource linter (and `black` itself) is (by design) ***not*** configurable. This forces aligning the existing options used by the tools invoked in `dev/linter.sh` (for 3rd party developers) with `BLACK` fbsource linting. Without this reconciliation, the different linters (used internally or by 3rd party developers) would simply conflict with each other resulting in artificial back-and-forth changes (for instance line width which `BLACK` forces to 88 characters).

Reviewed By: nikhilaravi

Differential Revision: D20558374

fbshipit-source-id: 614fa00664f8eb9d2de7438c29b807dfbf36ad20