.arcconfig

py3: fix sorting of obsolete markers during push

This fixes similar errors as in 01e8eefd9434:

  TypeError: '<' not supported between instances of 'NoneType' and 'tuple'

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