.arcconfig

automation: install black

This should unblock us from running the code formatting test in
our automated environment.

Differential Revision: https://phab.mercurial-scm.org/D7197

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}