.arcconfig

tests: use inline Python instead of sed to add trailing whitespace

The sed invocation was failing on OS X and FreeBSD. I'm far too lazy
to diagnose that, so just use some inline Python to fix the build.

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