contrib: add a fork of black (as "grey") that includes my changes This is black with https://github.com/psf/black/pull/826 applied as of today. The current git hash of black master is d9e71a75ccfefa3d9156a64c03313a0d4ad981e5, and the hash of my commit is dc1add6e94e212eff37bb3619e1422fb3c6d8dc8. In order to use this, you need to install `black` (from github master) and `typed-ast` using pip, preferably into python3, and then you can run `grey.py` with that Python and you'll have my patched version of black, which is how we've been formatting the codebase. Once my PR is merged, I'll follow up by removing this fork and updating instructions in the example config. # no-check-commit bad style Differential Revision: https://phab.mercurial-scm.org/D7002