.arcconfig

py3: ensure _start_response() is called with system string

This was preventing HTTP 500's from being sent in Python 3.

Differential Revision: https://phab.mercurial-scm.org/D4730

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