.arcconfig

changelog: use attrs instead of namedtuple

See http://www.attrs.org/en/stable/why.html#namedtuples for why attrs are
better than namedtuples.

Differential Revision: https://phab.mercurial-scm.org/D868

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    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}